It's any time. This lets you perform various shenanigans if you own both companies.

A couple of other things:

1. Remember that you can only buy trains, not sell them, on a given company's turn.

2. You can buy any number of trains up to the current train limit from anywhere that consents to sell them to you: this does include other players, but is very rare for it to be in two players' interests to participate in such a sale.

3. A common trick is to buy a train that's about to go obsolete to shift some money. For instance, if I have the CPR with $1000, I can buy, say, the NYC's three-train for $369, and then immediately buy a 6-train making the 3-train obsolete. This can be very helpful.

Train flipping is one of the essential "tricks" you must master to play this game effectively, and is one of the major reasons that you strive to own two companies at some point in time (the timing of which is also one of the necessary skills).

For example, while the Canadian Pacific Railway can conceivably be a viable line on its own (if you were ever able to set up a decent diesel run for it, it would be because your opponents have been asleep at the switch), its primary purpose is to provide the capital to properly outfit your "main" line. It is arguably why the late starting lines are there in the first place, from a gaming standpoint.