My daughter has had a Thomas set since she was one, 3.5 years now and she still she loves it, just bought dome track and a bridge from Ikea you can use it with the Thomas wooden set it is much cheaper, so you could get the generic tracks and use the Thomas trains, wish I had looked at IKEA's tracks before.
The nifty thing brio do is sell small packs of double ended pieces so if you are constructing a work of genius and end up with two holes or two pointy bits facing each other you can sort it out without taking the whole thing apart!
The Thomas wooden railway trains will fit exactly onto brio/ikea/bigjigs/generic wooden track - they're all the same guage. You sometimes need to give the ikea bits a shove to fit them with the other types but they do fit.
The cast iron thomas "take along" friends don't quite fit on any of the types of track exactly but that has never stopped DS using them on it happily.
I bought DS this set two years ago for Christmas (when he was 2). He still plays with it and the beauty is of course you don't need to assemble that full set each time - it comes in a lovely wooden box with rope handles and he can just make as much or as little as he fancies, or I help him make the whole thing.