Here's GuardDiver - it's a police car, fire truck and ambulance that can combine into train or robot modes (They're launched in train mode, then split into separate vehicles and can recombine into a robot).
I can easily see Toei going with a mixed theme, rather than just pure trains, so something like these last three robots might be possible.
There's also this, an anime starting up soon that, as the title should make obvious, is about trains. I can imagine Toei competing with this in some shape over the next year with Tokkyuger.
Well isn't that the 3 - 5 age group which I recall being Sentai's target audience as well?
Given that it's trains, it'll be a shame if Ishimaru Kenjiro doesn't have at least a cameo. Besides being Owner, he's also hosted a lot of shows about trains and train travel (I'm assuming this is probably why he was on Den-O).