See the Transport Fever / Transport Fever 2 series for what I would consider the "modern" 3D equivalent... apart from the fancy graphics you also have individually simulated passengers and more freedoms in terms of track/tunnel/bridge layout, and that combined with the rail speed limit mechanics is great, but:
* no multiplayer or AI, this is all solo/sandbox play... same thing as in TTD, you're economically limited at the start, but after a certain point you just have more money than you could ever spend
* the game is performance limited, once you've grow your network and the towns on your map large enough, first the 2/3x speeds slow down to normal speed, and then eventually even that starts lagging. You can still get 100h+ playtime out of a normal map on a decent computer before it starts becoming too much of an issue, though.
* Bridge building is fairly finicky, RIP "bridge pillar collision". OpenTTD is way more limited but also way more predictable.
All in all, TTD/OpenTTD still has a lot going for it 25-30 years later, and I'll happily play both .