Key takeaways
- The EM-SAT measures, it doesn't build: it determines the actual track position and supplies the correction values for the tamping machine.
- Absolute rather than relative: the reference is fixed survey points, not just the track itself.
- Fastest way to recognise it: a two-part combination of base vehicle and leading measuring wagon with no working units.
- No sensible tamping without a survey – the measuring run is the step before the actual construction work.
- Keeper and deployment locations change with construction programmes and clients.
Contents
Fact file
| Vehicle type | Self-propelled track surveying vehicle (engineering vehicle) |
|---|---|
| Manufacturer | Plasser & Theurer |
| Task | Determining the absolute track position and calculating the lift and line correction values for the tamping machine |
| Measuring principle | Long-base measurement between the base vehicle and a leading measuring wagon (satellite) |
| Reference | Fixed survey points along the track, not just the track's position relative to itself |
| UIC type code | 99 – auxiliary vehicle / special vehicle |
Identifying features
Is this really the class in front of you?
- Compact traction unit body with large windows and workstations inside
- Separate measuring wagon that runs on the track ahead of the base vehicle and is linked to it over a measuring base
- Optical measuring equipment between the base vehicle and the satellite – no working units in the ballast
- Warning livery with contrast stripes, beacon lights, work floodlights
- No tamping picks, no screening plant, no plough shares – the machine doesn't work on anything
- Twelve-digit vehicle number with the type code 99
Updated: August 2026 – Before a track can be corrected, someone needs to know where it actually lies. That’s exactly what the EM-SAT 120 is for: a track surveying vehicle that determines the absolute position of the track and calculates the lift and line correction values for the tamping machine from it. This page explains the survey principle and shows how to tell a measuring vehicle apart from a working machine.
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What is the Plasser & Theurer EM-SAT 120?
The EM-SAT 120 is a self-propelled track surveying vehicle. It doesn’t touch the track superstructure: no tamping picks, no screening plant, no plough shares. Its output is data – specifically the deviation of the track from its target position and the correction values derived from that.
Its two-part way of working is distinctive. A measuring wagon belongs to the base vehicle and runs on the track ahead of it. A long measuring base is set up between the two, over which the vehicle captures the position of the track. This design makes the EM-SAT one of the most unusual sights in engineering service: a vehicle travelling with a small wagon in tow, or rather ahead of it, without that wagon carrying anything at all.
Relative or absolute: the core problem of track surveying
A tamping machine classically measures with a tensioned chord running the length of the machine itself. That lets it detect whether the track deviates at one point from the line connecting two other points – in other words, local kinks and short waves. What it can’t, by design, detect: a long, gradual shift where the entire track deviates from the actual route axis over hundreds of metres. To the chord, a uniformly shifted track looks exactly like a correct one.
The blind spot that absolute measurement fixes
A track can lie perfectly evenly in itself and still be in the wrong place. Relative measuring methods never notice this, because their reference is the track itself. Only referencing fixed points makes it visible whether the route as a whole is still correct.
This is exactly where the EM-SAT comes in: it references its measurement to fixed survey points along the route, which are surveyed in advance by railway engineering firms such as IGEBA Ingenieurgesellschaft Bahn. That lets it say not only where the track is uneven, but also where it needs to be realigned as a whole.
How a measuring run works
Set up the measuring base
The measuring wagon is positioned ahead of the base vehicle. A long, optically monitored measuring path forms between the two.
Connect to fixed points
The measurement is tied to fixed survey points along the route. They provide the absolute reference that the track itself can't supply.
Capture the actual position
While the vehicle runs, the actual position of the track in height and alignment is recorded continuously.
Calculate correction values
Comparing actual and target position produces lift and line correction values for every measuring point.
Hand over the data
The values go to the tamping machine, which then works not to its own chord measurement, but to the target geometry.
Rare vehicles, rare chances
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Measuring isn’t all the same
Railway operation has several types of measuring vehicle that are easily confused. They differ in speed, what they measure and purpose.
| Vehicle type | What is measured | Typical way of working |
|---|---|---|
| Work-site surveying vehicle | absolute track position in height and alignment | slow, with a measuring wagon, ahead of the work site |
| Network measuring train | numerous track superstructure parameters across the network | at high speed during normal operation |
| Overhead line measuring vehicle | position and condition of the contact wire | measuring run under power, often with a measuring pantograph |
| Ultrasonic rail testing | internal rail defects | slow test run with test wheels on the rail |
| Radio and signalling measurement | transmission quality of the line equipment | measuring run with antenna and receiver technology |
The classic German measuring vehicles such as the DB Class 719 also belong to this family. The EM-SAT, by contrast, is purely a work-site vehicle – it doesn’t measure the network, but the section where work is coming up next. Its values then go straight to machines such as the Plasser & Theurer 08-275/4 ZW-Y or the Plasser & Theurer 09-4X Dynamic.
What the EM-SAT supplies is therefore the basis for everything that follows: without reliable measured values, a tamping machine could still work, but only relative to itself – and would carry a long, gradual positional error along unnoticed. That’s why measuring runs come at the start of a construction project, not the end.
Markings and what changes
Measuring vehicles are also auxiliary vehicles and carry a twelve-digit vehicle number, whose first two digits are the UIC type code – 99 for auxiliary vehicles. Country code, vehicle-specific part and self-check digit follow. How this number is structured and how to work out the check digit yourself is explained in the guide to the UIC number and its check digit.
A quirk arises with the EM-SAT from its two-part nature: base vehicle and measuring wagon are independent vehicles and each carries its own number. Anyone logging a sighting should therefore note which of the two they actually managed to read – otherwise it can later look like two independent observations, or like a single vehicle with contradictory numbers.
The data panel also matters for identifying it in the field: manufacturer designation, internal machine number and keeper are usually clearly legible on the body. Everything on it except the vehicle number can change. How to reliably capture such markings in photos and evaluate them later is covered in the article on markings and numbers in photos; the general approach to identifying unknown vehicles is in the guide to identifying classes.
A measuring run is still work-site operation
Measuring vehicles move slowly, but without warning, often on a possessed track and in work-site areas. You must not enter tracks, work sites or operational railway land. Only observe from publicly accessible locations – a measuring vehicle is no reason to get closer.
Common mix-ups
It's a surveying vehicle if …
- no working units at all are operating in the ballast or on the rail
- a small measuring wagon runs along ahead of or behind the vehicle
- the body contains workstations with windows rather than machine equipment
- the run is slow and shows no visible material movement
Look more closely if …
- the vehicle is travelling alone without a measuring wagon
- a tower wagon with a work platform looks similarly compact
- a rail trolley with a loading area looks confusable
It's not a surveying vehicle if …
- tamping picks or a lift-and-line unit are fitted
- ballast is being moved, screened or distributed
- a crane jib or attachment is mounted
Verdict
The EM-SAT 120 is the vehicle you overlook because it visibly does nothing – and without which the whole chain behind it would work noticeably worse. It answers the question that no tamping machine can answer on its own: not just whether the track is level in itself, but whether it’s in the right place at all. You can recognise it by its two-part combination and by the fact that it carries no working units whatsoever.
In short
Measuring, not building: the EM-SAT 120 determines the absolute track position over a long measuring base with a satellite wagon and supplies the lift and line correction values for the tamping machine. Vehicle number with type code 99, no working units on the vehicle.
Document what almost nobody logs
Measuring vehicles turn up irregularly and without warning – with Traintrack, every sighting is kept.
Summary
- Track surveying is the basis of all maintenance: only once the actual position is known can it be corrected.
- The EM-SAT works with a long measuring base between the base vehicle and the satellite wagon.
- Its output is lift and line correction values, which are handed directly to the tamping machine.
- You can recognise it by having no working units at all in the track – it only touches the rail.
Frequently asked questions
What does the EM-SAT 120 do?
It surveys the track. Unlike the classic chord measurement on a tamping machine, it determines the absolute position of the track relative to fixed survey points along the route. From comparing the actual and target positions it calculates the lift and line correction values that the tamping machine then works with.
What's the difference between relative and absolute track position?
Relative measurement compares the track with itself: it detects kinks and short waves, but not a long, gradual shift of the entire axis. Absolute measurement relates the track to fixed points outside it, so it also detects when an entire section is out of alignment sideways or in height from the intended route.
What's the separate measuring wagon for?
It sets up the measuring base. The satellite wagon runs ahead of the base vehicle; an optical measuring path runs between the two. The longer this base, the better it can capture long-wave positional errors that remain invisible with a short measuring chord.
Is the EM-SAT a construction train or a measuring train?
A measuring vehicle. It has no working units, doesn't touch the ballast and doesn't alter the track. Its output is data. In the sequence of a work site it therefore stands ahead of the working machines – and often again afterwards for verification.
How do the measured values reach the tamping machine?
As a data set. The calculated lift and line correction values are provided for each measuring point and used in the tamping machine's control system. That means the machine tamps not just to its own chord measurement, but to the previously determined target geometry – that's the decisive gain in quality.
How do I read the type designation?
It comes from the manufacturer's own system and describes the machine family and design, not a class in the sense of locomotive designations. The unique identifier is the twelve-digit vehicle number, whose first two digits are 99 for auxiliary vehicles.
Where am I most likely to see a track surveying vehicle?
On transfer runs and ahead of work sites, occasionally also during the day, because a measuring run needs less possession time than actual construction work. There are no fixed locations – keeper, markings and area of operation change with contracts and construction programmes.
How do I tell the EM-SAT apart from a network measuring train?
By size and purpose. Large network measuring trains run at line speed and capture numerous parameters during normal operation. The EM-SAT is a compact work-site vehicle that works slowly, runs with a satellite wagon and specifically supplies the input for the tamping that follows.
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Related classes
DB Class 719
Class 719 belongs to DB's measuring and survey vehicles. It checks track, overhead line and installations during regular operation – mostly unannounced.
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