Can I mix Fastrack and old track? Yes, you can mix Lionel Fastrack with older types of Lionel track.
There are advantages to this, especially if you have a bunch of the older track. Not everyone can afford to just throw away their old track and replace it all at once. Also, in some cases there are curved sections or switches that Lionel never made as Fastrack. But with transition pieces, you can use the curved pieces from older types of track. This can allow you to make a layout that wouldn’t otherwise fit in your available space.
The Lionel Fastrack transition piece has pins for traditional Lionel tububar O31 track, so you can use those two track types together. That adapter also allows Fastrack to work with other O31-compatible track types, like Supersnap track.
If you want to mix Fastrack with O27 type track, you’ll have to modify a Fastrack section on your own to make a transition piece. Pry up the tabs on one end of the Fastrack and pry up the rails enough to let you remove the standard pins. Next, take three O27 pins and solder the pins inside the Fastrack section. Finally, snap the rails back into place and twist the tabs enough that they will stay in place. Now you can connect a piece of O27 track to the newer Fastrack.
Of course, if you’d rather not pay for a pre-made O31 transition piece, you can solder O31 pins into a piece of Fastrack yourself. Then you have a choice of length and curvature.
To make the older track look better with Fastrack, you may want to add extra ties. After you do this, the difference between the two types of track becomes less jarring.