>>56OpenTTD is about managing all sorts of transport industries. Railroad Tycoon is a business management game for railroads. So, you don't manage bus routes or airports or ports or anything.
The business aspect is more in-depth, with stocks and bonds being a big part of play. There are no generic loans that you take out or pay off $20k at a time, instead you have the aforementioned bonds. Playing the stock market can net you and your company/companies money and/or sink your competitors.
Upkeep is also more in-depth. You don't need to fuel your trains directly, but fuel costs can be a major expense. Trains need to stop for oil, sand, and also water if they're steam. But it's otherwise not so in-depth as to detract from the experience. It's just enough to feel like you're maintaining a railway instead of OpenTTD's very hand-off feeling of upkeep.
Railroad Tycoon 2 is more like OpenTTD in that goods do not move without you transporting them, so a steel mill will never produce steel until you've given it coal and iron. Railroad Tycoon 3 lets goods move on their own, but very slowly, so you may still end up transporting said goods, but it's not always necessary.
Also, the music is great. Very comfy blues-y tunes, classic American railroad music. As much as I enjoy good MIDI music, this is all actual recordings. Even better, the files are loose MP3s so you can listen to them on your own, and you can take 2's soundtrack and add it to 3.
Train Fever was an overall disappointment in every regard, sadly. The music was great, though, if a tad limited.