Just in case it's remotely of interest this is one of the lamps taken out of it's housing.
It is of course of enormous interest, since you have started a thread asking about this problem, but you have been strangely starving us of information, making everyone guess and speculate, as you can see above.
Is it perhaps the case the bulbs you are trying to replace are similar to this one?
https://www.proflamps.com/us/en/Products/Applications/Airfield-%26-Railway/p/13800306842296/?currency=GBP
Given that the life expectancy is only 600 hours, I can see why replacing them could become annoying. On the other hand, given that the indicated price is £1.45 per bulb, and each bulb will have to be replaced every 25 days, you will need something very inexpensive as a replacement to provide a reasonable payback on the investment, I would think?
It is not just the costs for the new lamps (which seem to be higher by a factor of 10 - though likely with a discount for larger quanteties). Often much of the costs are for sending out a maintainance team to sometimes relatively remore locations. At least less ladders needed for the rail signals.
When introducing the LED trafic lights the longer service intervals were a big plus, not so much the energy savings.
Depending on the environment one may still need regular cleaning (e.g. check for spider webs).
Modern railways have an electronic signal going to the train as a backup. The optical signal is limited reliability becaus of the human factor anyway, no matter how good the lamps.
Normally I would consider lamps with 3 redundant strings and than a somewhat more sensitive system to detect a drop in the current good enough. 2/3 the intensity should be good enough until fixed.
A failure with reduced optical power when getting the right current is rare and mainly an issue when running too hot or a series failure with white LED loosing the phosphor. Even if 1 LED in the string would fail, this would be something like 1 in 3 or 4.
Modulation of the lamps would be a way to make it easier for an electgronic system to detect them too - but that would be a larger change to the system.
Chances are this would need changes in the regulations anyway, no matter how small the change.