DS had the first bits of his wooden train set for his 2nd birthday. he is now nearly 6, and we have added quite a lot to it over the last few years, as he is trains obsessed. We have bought Big Jigs (careful of your spelling when you google that as the 'i' is next to the 'u' on the keypad!), Brio and Thomas track and stations, bridges, clock tower, windmill, engine shed, repair shed, signal, level crossing etc etc.
The Thomas brand is probably the most expensive. But all of the brands are compatible. Big Jigs is amazing value for money. DS' set has been played with daily, and none of it has broken, so my advice would be to get the Thomas trains as they are instantly recognisable, and get the track, stations, bridges etc from Brio or Big Jigs or Toys R Us own brand or whatever.
Special mention for the talking Brio station, which is fantastic (Ds records his voice making announcements!), Toby the Windmill and The double signal and the deluxe level crossing and the repair shed from the Thomas range, and the engine shed and wash down from the Big Jigs range. These risers are great too for buildinf intersting layouts, but I have no idea why Amazon have them priced at £78 - we paid about £6 for ours!
Get loads of track, including hills, points, curves (long and short) and lots of different sized straight pieces. Then you can create complicated routes and the trains can go under each other.
I think its Brio who do raod pieces (some of the Thomas track has a road layout underneath too) which you can buy to go with the train track to create more of a scene.
Enjoy!