Is building a few gantries Which last for decades really cheaper than building all the balises, plus all the extra equipment every single New train has to have?
And is fixed line side equipment really less reliable than hundreds of sets of equipment bouncing around on trains and relying on radio links?
I think you mean ‘really more expensive’. The Balise are all the same except for simple ID programming that would be done in a workshop before fitting to the track. They have no power and are thus more reliable, also if the train misses one Balise that’s ok provided it doesn’t miss the next one, improving system availability. Adding the on-board equipment as standard reduces unit cost due to the volumes involved, also there is in-built redundancy of the electronics with (possibly, but I’m not involved these days being retired) a two out of three channel configuration, much like SSI. Along with other on train systems faults will be recorded as they happen and increasingly reported direct to a maintenance depot for rectification later.
Reliability of lineside equipment may not be the critical issue. Consider a track circuit fault, let’s say through a tunnel. Is the location of the fault obvious, can technicians get to it quickly, have they gone to the right end, has the equipment failed or is there a track fault such as a failed insulated rail joint. Such failures present logistical issues associated with what are known as ‘single point failures’. All the time the fault exists trains are at a stand or heavily delayed, particularly if a set of points is locked by the track circuit being ‘occupied’.
Note that axle counters systems can be designed with redundancy to overcome that ‘single point failure’ problem, and remote monitoring is available.
It all comes back to RAMS, Reliability, Availability, Maintainability and Safety. All these elements of design can be calculated (having been involved in such work, I can confirm it can be complex). With careful attention to RAMS there is no reason why on-board equipment would be less available than any other equipment.
Note that I have experience of both on train and fixed signalling systems going back over thirty years. Designing a system with automatically driven trains, (including in depots), in tunnel throughout, unattended (i.e. no staff on board) on a close headway mass transit railway in Singapore tends to focus the mind on RAMS where availability levels of 99.9% are required.
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