You raise some genuine possible concerns but generically I'd probably tend to just say, "laws will stop them" just like they already stop someone deliberately endangering lives by placing a real fake stop light in the middle of the 101 at rush hour.
The real concern would be whether someone can engineer a terrorist level mass scale attack but as long as it requires physical tampering that adds up to a tremendous amount of work. So if the signalling is largely burned into fixed infastructure it eliminates a lot of that or at least sets the bar high enough that its probably more work than various other types of attack that are likely to be just as impactful.
