Most train games are good for a fairly small range of players. By looking at two games which do seem to scale well we can see what mechanism help scalability (but it's really just an excuse to gush over 2 games I really like). On the Underground player 2-5 and 1825 plays 2-9 (although I've only played up to 5).
There are many way to get victory points in On the Underground: loops, passenger, National Rail stations, connecting two matching stations, and terminus stations. That's an awful lot of different "paths to victory" available. If someone is blocking one VP source then there are so many others to choose from. This game gets its tension from providing an environment where you want to do 5 or 6 things each turn, but can only do 4.
The game plays in semi-random number of player turns (there is a deck from which 1 or 2 cards are drawn each turn and when exhausted it's the final round). This keeps games at a similar length regardless of how many players are in the game. There is more downtime with more players, but since so much of the game is trying to connect lines without being blocked and blocking others, I am involved all the time.
The other scalability mechanism is giving out different amounts of track lengths depending on the number of players. Even the number of different colours you have available changes (and since you can only add to ends of existing lines, number of colours does change the opportunities available on the board).
It does feel like the same game with 2, 4, and 5 players (I have not played the 3 player version, yet). I think this is, in part, due to the passenger movement. There is always something out of your control each turn so adding chaos from other player's moves doesn't seem like a major shift.
1825 on the other hand, only has "one" way to get "points": money in hand at the end of the game. It scales, in part, because there are more companies than players. You can (and I usually do) run 2 or 3 companies in the game.
Another huge scaling factor in 1825 is the modularity of the board. You play on different parts of Britain depending on how many players. This keeps the number of companies per player manageable, and keeps track building competition intense.
The joy of 1825 comes from setting up a long term business plan for your companies and watching them fail. Also watching the stock market for undervalued stock is a must. 1825 is short enough (with 2-5 players) to play in an evening, and with the linear stock market is less overtly cutthroat than the 1830 branch of 18xx.