After a while you will have a new kind of train available and you will need to build a new kind of track to be able to make them run. The track is one rail instead of two. To access them, I beleive you need to click on the train icon and choose either for regular trains or the new kind.
I personally hardly ever played with those trains because I'm so fond in trying to break the record of
The Transport Tycoon Deluxe Challenge. This mean that I usually stop playing way before 1990...
By what I remember, those trains are really faster tough you
will need to change every track if you want to switch to them. There is no way around this.