So, what you basicly do is build a railroad before 1900 and get some trains running on them. As time moves on and the trains get older (and more expensive and slower compared to more modern trains) you send 1 train back to the depot and keep it there until the year 2000. Don’t keep it running on your tracks or on a line. All other trains on that line can then be upgraded as per usual and the old, slow train will not back up your other trains.
Once you reach Y2K you simply pull that train out of the depot by sending it to a line again. The moment that train has left the depot, the achievment should be unlocked and you can send it back and sell it off.
Another way of doing it is by simply keeping a museum line running where you can keep (if you would like) multiple old trains running.