Key takeaways
- Class 664 is a diesel-electric mainline locomotive of American EMD design, procured in the Yugoslav era.
- Quickest route to identification: no pantograph, number starting with 6, a stand-alone locomotive rather than a multiple unit.
- Against Class 661, the number decides – designs from the same manufacturer family look very much alike.
- Its territory is lines without overhead wires, plus transfer runs, engineering and works trains on the electrified network.
- Fleet and duties are constantly changing, as older diesel generations are gradually replaced.
Contents
Fact file
| Vehicle type | Diesel-electric mainline locomotive for freight and passenger duties |
|---|---|
| Design origin | General Motors / EMD, United States |
| Procurement | For the Yugoslav state railway JŽ, after 1991 partly with Slovenske železnice |
| Drive | Diesel engine with electric transmission to the traction motors |
| Class number | 664 – at SŽ, the 6xx block denotes diesel traction |
| Typical area of operation | Non-electrified lines, transfer runs, engineering and works trains |
| UIC country code for Slovenia | 79 |
Identifying features
Is this really the class in front of you?
- No pantograph on the roof, but an exhaust and large radiator structures instead
- Class number starts with 664; the leading 6 stands for diesel traction
- Body shape follows an American design lineage – proportioned differently from European diesel locomotives of the same era
- Often found on lines without overhead wires or on tracks that are not wired throughout
- A separate locomotive with its own rake of coaches or wagons – not a through multiple unit
- Liveries have changed over the decades and are no use as an identification feature
Updated: August 2026 – Where there are no overhead wires in Slovenia, diesel traction takes over – and that includes Class 664, a diesel-electric mainline locomotive from an American design tradition. It’s not a vehicle you tick off in passing in Germany, but one that carries an entire procurement history with it. This page shows you how to place it and where you can realistically find it.
Spotted a diesel abroad? Secure the number.
Slovenian diesel classes look alike from the outside – the markings decide. Record them in Traintrack.
What is Class 664?
SŽ Class 664 is a diesel-electric mainline locomotive whose design goes back to General Motors and its locomotive division EMD. It was procured for the Yugoslav state railway JŽ; after 1991 part of the fleet remained with Slovenske železnice. Other machines of American origin stayed in Croatia, where HŽ Putnički prijevoz runs passenger services today.
Diesel-electric means: the diesel engine drives a generator, which supplies the traction motors on the axles. There is no mechanical gearbox. From the outside you can recognise this principle by the large radiator structures, which are essential on a machine of this power class – and by the deep, even engine sound, which is clearly different from small diesels with hydraulic transmission.
Why American diesel locomotives run in Slovenia
The explanation lies in the procurement policy of the Yugoslav era. While the Western European railways ordered their diesel fleets from domestic manufacturers, Yugoslavia turned to American designs on a large scale. The result is a fleet that comes, technically and visually, from a different engineering tradition from anything built in Germany.
For spotters, that’s the real appeal. If you come from Germany, you know European-design diesel locomotives in every variant. A machine with an American lineage in a Central European station, on the other hand, is a subject that simply doesn’t exist at home. How to capture such differences systematically, rather than just marvelling at them, is described in the guide to identifying classes.
First settle diesel or electric
No pantograph, but an exhaust and radiator surfaces: that rules out all 3xx classes and the modern interoperable locomotives.
Tell locomotive from multiple unit
A stand-alone locomotive with a rake of coaches or wagons attached is something different from a through multiple unit. That decides between 6xx locomotive classes and railcar classes.
Read the class number
664 on the side of the vehicle is the only reliable way to separate it from the related classes of the same manufacturer family.
Note the UIC number
The country code 79 confirms Slovenian registration. Together with the serial number and check digit, the sighting is then unambiguous.
Where the electrification gap determines deployment
The field of operation of a mainline diesel locomotive follows almost entirely from the question of where there are no overhead wires, or where they can’t be used.
| Situation | Why diesel is needed | What you can observe |
|---|---|---|
| Non-electrified lines | Electric traction technically impossible | Scheduled duties, reasonably predictable |
| Transfer runs to private sidings | Sidings mostly without overhead lines | Short trips, as required |
| Engineering and works trains | Overhead line switched off during engineering works | Irregular, often in unusual places |
| Shunting within station areas | Not every track is wired | Recurring movement patterns, easy to observe |
| Standing in during disruptions | Independence from the power supply as an advantage | Rare and impossible to plan for |
Within Slovenian diesel traction, the classes share out the work: Class 661, like the 664, belongs to the heavier category, while Class 643 tends to cover shunting and light mainline work. On the electrified part of the network, by contrast, machines such as the six-axle Class 363 do the work – which can’t move a single metre without overhead wires.
Area of operation and fleet are subject to change
Which diesel class currently works which duty changes with tenders, timetable years and vehicle availability. Older overviews from the internet go out of date particularly quickly for this vehicle category. Check the situation shortly before your trip.
Current sightings instead of old lists
See which Slovenian vehicles were reported recently – and plan your tour accordingly.
Spotting with a telephoto: distance is the norm
With diesel locomotives on works duty, you rarely get close. The interesting areas lie behind barriers, and there’s no way round that. The solution isn’t to get closer but to plan better.
- Be generous with focal length. If your position is fixed, the lens has to bridge the distance. Which telephoto focal lengths prove themselves in practice, and where cheap optics reach their limits, is covered in the guide to the best telephoto lens for railway photography.
- Look for elevated positions. Bridges and publicly accessible overpasses give a clear view over track layouts – without you having to approach the operational railway land.
- Observe first, then photograph. Shunting and transfer moves repeat themselves. Watching one round costs ten minutes and saves you three failed shots.
- Log sightings immediately. Number, location, time and activity – after two days of travelling, you’ll otherwise no longer know which machine stood where.
If you want to deepen your research before the trip, the article on railway topics abroad points you to where international sightings are documented and discussed.
One final point concerns trip planning. Non-electrified lines are almost always away from the main corridors, and public transport to them is sparse. If you head for destinations like these, calculate your journey realistically and plan a return route that still works if your stay runs late. A subject you can only reach with a tight connection is rarely worth the risk – and in the meantime, the locomotive you’re after may well be standing at a larger station anyway, where you can take photos comfortably and safely.
Safety is not negotiable
Tracks, storage sidings and depots are operational railway land and, as a rule, private property. Only take photos from publicly accessible locations, keep your distance from the platform edge and never enter track areas – not even for a supposedly once-in-a-lifetime shot.
Common mix-ups
It points to Class 664 if …
- there is no pantograph, but an exhaust and large radiator surfaces instead
- it is a stand-alone locomotive with a train attached, not a multiple unit
- the class number starts with 664
- the vehicle is running on a non-electrified section
Look more closely if …
- a similar-looking diesel locomotive is standing next to it – the EMD family is visually closely related
- the markings are not legible
- you only see the vehicle from a long way off or against the light
It isn't a 664 if …
- a pantograph is touching the overhead wire
- the unit can be walked through from end to end and has no separate locomotive
- the class number starts with 3, 7, 193 or 541
Verdict
Class 664 stands for a part of the Slovenian railway that works without much attention: traction where the overhead wires end. Its American origins make it a genuine travel subject for German spotters, and its similarity to its sister classes makes it a diligence exercise – without a legible number, every identification remains a guess. Bring the two together and you have a sighting that will still stand up in ten years’ time.
In short
No pantograph, large radiator surfaces, a stand-alone locomotive, number 664: that's how you place the Slovenian mainline diesel. Its territory is lines without overhead wires – and against the related classes, the class number is the only reliable feature.
A collection built on numbers, not guesses
Record class number, UIC number, location and date together – that keeps your Slovenia collection verifiable.
Summary
- Class 664 belongs to the heavy diesel traction of the Slovenian network and comes from an American design tradition.
- The Slovenian numbering system clearly assigns it to diesel traction via the 6xx block.
- Its field of operation follows from the electrification gap: where there are no overhead wires, diesel is needed.
- For documentation, the full number, date and location count – the class number alone is not enough.
Frequently asked questions
What is SŽ Class 664?
A diesel-electric mainline locomotive on the Slovenian network whose design goes back to General Motors, or rather EMD. It was procured for the Yugoslav state railway; after Slovenian independence, a share remained with Slovenske železnice. The design is used where electric traction is not possible.
How do I reliably identify Class 664?
In two steps. First, the roof: no pantograph, but an exhaust and radiator structures – that rules out every electric locomotive. Second, the class number on the side of the vehicle. Because the Slovenian diesel classes come from the same manufacturer family and look similar from the outside, the number is especially important here.
What does diesel-electric mean?
The diesel engine does not drive a gear train but a generator, which in turn feeds the traction motors on the axles. This principle is widespread among heavy diesel locomotives worldwide because it transmits high power without a mechanical gearbox. From the outside, you can recognise it by the large radiator surfaces that have to dissipate the waste heat.
Why does an electrified country need diesel locomotives?
Because no network is fully electrified. Branch lines, sidings, goods loading roads and depot areas often have no overhead wires, and the overhead line is switched off during engineering works. Diesel locomotives work independently of all that – which is why every railway company keeps a diesel fleet, even when the main lines have long been electrified.
Does Class 664 run in Germany?
No. The class was procured for Slovenian, formerly Yugoslav, operation and is neither approved nor to be found in Germany. For German spotters that means: these vehicles are a travel destination, not a chance find on the local line.
How do I tell the 664 apart from Class 661?
By the class number. Both classes come from the same American design tradition, and the overall visual impression is not enough for a reliable identification. A photo with legible markings is therefore more important with these classes than with European designs, which can already be told apart by their front shape.
Where is it worth looking for Class 664?
On and alongside lines without overhead wires, at stations where electrified and non-electrified sections meet, and around engineering works. Which duties are currently worked by which class changes with timetable years and vehicle availability – so plan with up-to-date information.
Is the class still in service today?
The condition of individual machines of an older diesel generation varies considerably, and a blanket statement about the current fleet would not be credible. Only current observation is reliable: sightings from the recent past tell you more than any older fleet list from the internet.
Log your sighting of the SZ 664
Date, location and vehicle number logged in seconds – and visible to the community.
Related classes
SZ 661
In Slovenia, Class 661 goes by the nickname Kennedy and comes from an American EMD design. Here's how to place it and how to find it on the ground.
Diesel locoSZ Class 643
The Class 643 is one of SŽ's diesel workhorses. Here's how to recognise it, how to make sense of the Slovenian numbering system and where to find it.
Electric locoSZ 363
Six axles, a sloping front, the nickname Brigitte: SŽ Class 363 is Slovenia's most distinctive electric locomotive. Here's how to recognise and find it.
Who is behind it
Groups, brands, manufacturers and operators – who owns whom and how to recognise them out in the field.
HŽ Putnički prijevoz
HŽ Putnički prijevoz is the passenger arm of the Croatian state railway – one of three sister companies since the split, alongside freight and the network. Its vehicle numbers follow a Yugoslav logic that's worth understanding once and for all.
State railwaySlovenske železnice (SŽ)
Slovenia is small, but sits on one of Europe's most important transit axes: from the port of Koper heading north. Slovenske železnice runs on 3 kV DC there – and the electrification system changes at three of the country's four borders. That shapes the locomotives more than anything else.
Related guides from the blog
