Hi! We've been collecting Thomas/Brio for about 7 years now and the best accessory ever I'd have to say is Cranky from Thomas. Both the kids are wild over him, he's easy to use but interesting enough that the children get a sense of accomplishment from mastering him.... and the magnetic feature is neat too, the kids get to see the magnet actually snap something up in mid air... they love that. He is usually a pricey toy (about 100 canadian) but I got mine at a second hand store for 35! I was going to buy him new, totally willing to pay out the big bucks for such a cool addition to the set, but then I found him used and thought EVEN BETTER!
I am next planning on purchasing the roundhouse... its the last of the big pieces that we want. Until they come up with some new thing, that is!
Oh yes, I should mention we started with the basic brio figure 8 set for about 50 bucks, and it was the best starting point we could have gone for. We still use all the pieces from it in our ever changing configuration.
We are blessed with more Thomas stuff than you would ever want, thanks to dh's godmother who loves to 'collect' things and decided that our kids 'should' collect Thomas things and that it was her job to provide them! (Hey, who am I to complain?!)
Our kids like:
The fire station
the cargo transfer station
bridges (especially the lift bridges)
In our experience, cranky the crane falls apart a bit too easily for my kids to be able to use him (the top comes off easily - it's not too hard to put back on, but my fine motor challenged son won't/can't do it).