On my son's second birthday we bought him a basic wooden train set "compatible with Brio". For the Christmas after that we added an expansion pack and a Gordon engine. At that age he was happy pushing the train round in circles or small figures of 8 (though his Dad and I competed to build complex layouts that allowed trains to go round either way).
On his 3rd birthday (and subsequent Christmas) we (and the grandparents) got him more train track, including Brio points, bridges, turntables etc and several of the other engines. At that age he wanted to act out the Thomas the Tank engine stories - so we'd read him a story (or he'd watch the video) and then he'd act it out on his layout.
On his 4th birthday (and subsequent Christmas) he asked for these roads, along with the level crossing (Brio Rail and Road Crossings BR33207), and a "German electric train" because "steam trains are from the olden days". So for his birthday he got the NEW BRIO 33209 RAIL & ROAD TRAVEL SET - but that had very little road, for Christmas he got this road expansion (and the similar curved road one).
He now likes to make complex layouts (more complicated than even Daddy's) and enact "real-life" stories. After a trip on the Wimbledon-Croydon tram link, this road was connected straight onto the train track (and it fits that way too) and his electric train became a tram.
So well done Brio - you've solved the question of "what to buy him" for 3 years in a row - and he has played with it for 3 years. Now, at 4 and a half, he's finally reducing how much he plays with this - and getting "into rugby" instead. But with his brother about to be born, I think we're going to have a few more years play from this.
For three years this is the toy that has kept him most occupied, and expanded with his developing creativity. He talks to himself whilst playing with it, telling wonderful stories. And the track is generally in very good condition, so I suspect that even after his brother plays with it for another 3 years, there will be "resale value" at the end.