Yeah, that's really the biggest issue with these systems. Were I one of these manufacturers I'd invest in getting the current software running in a VM specifically to make it easier to transition to new hardware. The last thing they'll want to deal with is that their software was written for Sun Spark and that no longer exists.
That said, hard to go wrong porting over the x86 at this point.
Some of the signal control infrastructure for the Melbourne rail network was ported over to an emulated PDP-11 System for this reason. This was previously discussed on HN here: https://news.ycombinator.com/item?id=42514553