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Frequently Asked Questions: T4D Rail

Trimble® 4D Control™ Rail module has emerged as a game-changing solution for keeping rail infrastructure safe. Demand for efficient and sustainable transportation grows, causing the same trend for the number of construction activities within the rail environment.

These answers to common questions surrounding rail monitoring and the T4D software add-on module, T4D Rail, provide an overview of key benefits, helpful features and potential project impact. 

Question 1: In what scenarios would you use the T4D Rail software module for rail monitoring instead of monitoring only with T4D software?

The answer to this question depends on the project requirements and desired deliverables. Typically, during rail monitoring projects, specific rail parameters are being monitored (refer to question 5 for more detail). Utilizing T4D without the T4D Rail module may require substantial manual effort and users would potentially encounter limitations. For projects exclusively requiring displacement data—such as monitoring a single prism on the track's profile—T4D alone may be sufficient. 

On the other hand, T4D Rail is a tailored solution for rail monitoring projects and provides specialized calculations, visualization, deliverables and features like as-built data integration and automated offset calculation, which streamlines the entire rail monitoring process. As an add-on software module for T4D, T4D Rail gives users the benefit of several combined functionalities and added customization tools adjustable to their unique needs.

Question 2: Is conducting a track survey a mandatory prerequisite for rail monitoring with T4D Rail?

The short answer is no, but it depends on the desired results. As-built data, as a result of a track survey, contains the coordinates of the rails, gauge, cant and station information. This data can be used to get from the measured prism to the point on the rail head (i.e., offset calculation (see image)). The exact coordinates of a rail track can then be used for the calculation of the rail parameters. Parameters calculated this way are then called absolute results since they represent the actual track geometry (e.g., absolute twist value). So, conducting a track survey allows users to monitor absolute track parameters and fully understand track movements.

offsets between prism and railhead
Offsets between the prism and the railhead

Question 3: What are the methods for efficiently collecting the as-built survey data for rail monitoring using T4D Rail?

There are two ways as-built data can be collected for later use in T4D Rail. The first tool is the Trimble GEDO System, which is used with the Trimble Access Rail app and the Rec or Track module within the app. Once the track is surveyed, if desired, data can be edited in the GEDO Office Module and from there, the *.track file (see question 4) can be exported. 

The second tool, the Trimble Access™ Track Gauge Survey app, is a cost-effective and user-friendly solution that works in conjunction with the track measurement bar. This solution is ideal for small surveying tasks or collecting data for monitoring purposes, and it allows users to export necessary data directly from the field. 

Question 4: What is the *.track file?

The *.track file is generated after a track survey (as-built survey) has been completed. This file is a 3D representation of the entire actual track position and contains all track geometry information necessary for a project. The *.track file is used in T4D Rail for the offset calculation between the measured prism and the point on the rail head, which then allows the monitoring of absolute parameter values. 

Question 5: What parameters can be calculated using T4D Rail?

T4D Rail supports calculations of the most commonly monitored rail parameters. See the definitions below:

  • Cant — Difference in elevation between the top of the two railheads at a chainage
  • ΔCant — Change in cant when compared to the reference measurement (epoch)
  • Twist — Difference in cant between two chainages
  • ΔTwist — Change in a twist when compared to the reference measurement (epoch)
  • Vertical displacement — Change in a track's elevation when compared to the reference measurement (epoch)
  • Horizontal displacement — Change in a track's position laterally when compared to the reference measurement (epoch)
  • Vertical versine — Variation in a track's elevation over a certain distance (chord length)
  • Horizontal versine — Variation in a track's horizontal position over a certain distance (chord length)
  • Horizontal versine displacement — Variation in track's horizontal displacements over a certain distance (chord length)

rail monitoring parameters
Rail monitoring parameters

Question 6: What is the difference between an automated monitoring total station (AMTS) set-up with and without integrated as-built survey data?

The main difference between an AMTS set-up with and without as-built data is the parameter calculation. 

For workflows where as-built data is available, with every new measurement, parameters are calculated using actual rail coordinates, which allows monitoring for absolute parameter values. 

Without as-built survey data, with every new measurement, rail parameters are calculated using prism position. Even though we can assume that the prism moves and behaves the same as the rail and the sleeper, the prism position and elevation do not represent the track's position and elevation.

As a result, the data collected does not allow for monitoring of absolute values (cant and twist). Versines calculated without the as-built data are versines from the prism position and do not represent the actual track geometry. Still, information on how they change can be valuable during the monitoring process and can be sufficient for specific monitoring projects (e.g., minor construction works beneath the track, such as drilling for cables or similar).

Question 7: Which specific types of prisms are compatible with T4D? What is recommended for rail monitoring, and how important is the correct installation?

All geodetic (surveying) prisms suitable for installation alongside a railway, or directly on the rail track itself, are compatible with T4D. It is crucial to ensure proper installation, as the prisms directly reflect the movement of the track. One best practice is to fix the prism onto the sleeper through drilling (if permitted) or adhesive application. Alternatively, for installation on the rail, choices are adhesive bonding, magnetic attachment or the use of rail clamps (see image).

Different prism installation options (from left to right: prism glued on the rail, glued on the sleeper, rail clamp)
Different prism installation options (from left to right: prism glued on the rail, glued on the sleeper and rail clamp)

Question 8: What happens if, for some reason, a prism is not measured during the monitoring period?

Given the potential for a high volume of train traffic in the monitored area, occasional disruptions in prism measurements are to be expected. 

The consequences of the prism not being measured during monitoring depend on the observation date parameter. This parameter gives information about how old the measurement is, which is used for parameter calculation. 

Should a specific prism be missing a measurement, and provided the alarm threshold of the observation age is not reached, T4D Rail strategically uses the most recent available measurement for that prism. 

This approach ensures seamless data flow, eliminating gaps in the results. At the same time, it offers insights into how old the utilized measurement is, enhancing transparency in the decision-making process. 

However, should the predefined observation age threshold be reached, T4D Rail suspends calculations, resulting in a temporary absence of data for specific areas. This prompts users to take action to either investigate or address the absence of new measurements for the designated prisms.

observation age parameter
Observation age parameter 

Question 9: Which sensors are compatible with the T4D ecosystem for rail applications?

T4D Rail supports measurements from total stations and tiltmeters. Total stations can be used as part of an automated or semi-automated (campaign-based or manual) monitoring system. They offer additional flexibility when it comes to as-built data availability, making rail monitoring possible without surveying the track, in case relative results are sufficient (parameters calculated from prism coordinates).

Tiltmeters offer another great sensor option and can be utilized as a complete solution. Use it for projects including as a pre-warning system for areas where line-of-sight is difficult, if there are challenging weather conditions (e.g., snow), or when the area to monitor is large. Tiltmeters also complement AMTS set-ups, ensuring continuous measurement flows regardless of weather or traffic conditions.

Additional sensors like weather stations, laser tiltmeters and vibration sensors are frequently utilized in the rail industry and are supported in T4D as well. This not only provides enhanced flexibility but, in certain instances, crucial insights for a complete understanding of movement.

Question 10: Is it possible to replace prisms in the software that have been damaged or destroyed in the field?

Yes, T4D supports a prism replacement workflow. If the prism was destroyed in the field, after physically replacing it, you can re-calculate the offsets between the newly installed prisms and the actual track position and continue the monitoring process without starting the project all over again. Note that this applies only to cases where the rail geometry has not changed. In the AMTS set-up with the as-built workflow, if the geometry has changed, a new as-built survey is necessary.

Question 11: How can you access the results from T4D Rail? How can customers with no T4D Rail license access the information?

There are two types of rail monitoring result files that can be downloaded for non-license holders. First, there's the *.xlsx file available for download, encompassing concise summary data on calculated parameters, along with results from every measured round across all chainages for each parameter. This file serves as the foundation for generating specialized rail reports. 

The second .xlsx file available for download is called Detailed Report. In addition to the already mentioned details, the Detailed Report provides comprehensive coordinates for measured prisms, rails and their deltas and calculated offsets. This richer dataset offers even more flexibility for creating specialized rail reports or, if necessary, showing where further calculations are needed.

Alternatively, stakeholders can always get a pulse on the data by gaining access to the monitoring project itself. This access can prove invaluable, especially in scenarios where change notifications are received. Stakeholders can effortlessly open the rail monitoring project in T4D Rail to visually pinpoint changes, comprehending the full scope of the situation. It is a seamless way to ensure constant awareness and understanding of changes in conditions.

Detailed report example

Should you have additional questions or suggestions, please don't hesitate to reach out. Visit the T4D Rail product page for more information on T4D.