Is Level 2 Overlay a good idea?

I was recently asked about the differences between overlay and underlay for ETCS and it set me thinking about how a railway decides to implement ETCS (if at all).

Most railways have relied on the driver observing lineside signals and signs in order to establish the speed the train can travel at and where it must not pass. Generally, this works well but humans are, well, human and mistakes can happen. Systems have been developed which reduce the likelihood of error and manage the consequences. Some are alerts to the driver to aid them in identifying there is a sign or signal to obey, whilst others provide a level of protection or supervision.

These systems generally involve an interface to the signalling system, normally at the signal, to control transmitters (beacons, loops, magnets or loops) situated between the rails. The interfaces vary, some merely monitor the controls to the signal, others provide more feedback to the signalling. The most common uses a lineside encoding/electronics unit (LEU) to generate the message to be transmitted to the train based on the signal aspect displayed.

In the UK, the national system is a little different, based on history, with the AWS alert and TPWS supervision being interfaced as a simple on/off control. However, on the GW and Chiltern routes the trial Automatic Train Protection systems use a form of LEU.

The signals themselves also vary with some systems providing information about the maximum speed the driver can travel at whereas others inform them how far (signal sections) they can travel, and sometimes a combination of the two. This means that, with time, a number of legacy protection systems with their own transmission methods and protocols, and associated LEUs have been developed. Also, as time has moved on, these legacy systems have become harder to maintain due to lack of manufacturing, obsolete components, etc.

All this brings us to the considerations that railways make about whether to adopt ETCS Level 1 which, if you do have a variety of legacy systems, based on LEUs, can be an attractive option. Where the LEU already drives balises then it may be possible to reconfigure the system to also transmit ETCS information from the same LEU and balise combination. Where the transmission uses a different system, adding a second LEU to drive ETCS balises is familiar design and installation techniques.

Adding the ETCS Level 1 functionality whist retaining the legacy system makes the migration much easier since the track can be equipped and then trains introduced or modified as part of a rolling programme. This leads one to the reasons why railways adopt ETCS Level 1. We have already touched on the problems associated with maintaining legacy systems and procuring and fitting the equipment to new or other trains.

Where trains operate over multiple routes, particularly freight or cargo flows, then the locomotives need to support all the relevant legacy systems which is costly and complex. Adopting a harmonised system makes a lot of sense simplifying the train fitment and also, over time, removing a maintenance and cost burden from the infrastructure manager.

Once trains are fitted with ETCS equipment it also enables their operation on new lines or upgraded lines fitted with ETCS Level 2 with the capacity opportunities that offers and the elimination of lineside signals.

If your legacy systems don’t involve LEUs or the nature of your trackside signalling does not readily support the use of Level 1 interfaces, then there may be advantages to using radio based ETCS. One of the challenges of Level 1 is managing multiple routes and types of movement from a signal. There are solutions but they are more complicated and costly.

Overlay ETCS Level 2 is, in its purest form, the ability to send to trains movement authorities which match the lineside aspects being displayed to the driver. Rather than using an LEU to extract the aspect information at the signal, the Radio Block Centre can obtain the information direct from the interlocking.

However, there are some challenges with using ETCS in this manner. The first is where your signal aspects convey speed information, something that Level 1 addresses by including a speed variable in the Level 1 Movement Authority. This is missing in Level 2 so other means may be required to control the speed if you want a 1-1 relationship between the cab display and the lineside aspects.

A second challenge is where signals are controlled by local, lineside circuits and not directly from the interlocking. Many railways only provide interlocking in stations and junctions with all the intermediate signals controlled automatically with cabling from signal to signal. In these arrangements, it is no longer as simple as adding an RBC to the interlocking but the entire signalling system needs to be replaced and suitable technologies may not be available.

Before moving on to different forms of overlay and underlay in a subsequent dissertation, we need to think about why railways (infrastructure manager and operators) might want to deploy ETCS Level 1 or overlay. As already discussed, a prime reason may to be to reduce and eliminate legacy systems. Another reason may be where the legacy system has limited supervision functions and needs to be enhanced to meet safety targets. If the long term strategy is to eliminate signals in order to provide more capacity or enable high speed operations, then the introduction of ETCS and the in cab displays may be seen as part of the driver training and familiarisation process.

ETCS Level 2 overlay is not a single, simple solution since it depends on why you are doing it and the signalling technologies currently in use.