Lionel Fastrack Uncoupler with older 1656 engine bottoms out on wheel flange.

[Minnegan]

Fastrack switches will be an issue to. My Locomotives with roller pickups have no problems, the sliders all hit badly on the FasTrack switches.

[oscar99]

Lionel postwar trains were designed to run on tubular track, and many postwar locomatives and accessories may not work well onFastrack.

[irontooth]

If possible I woud check other Fastrack uncouplers to see if they are the same or if the magnet in lower. My old Lionel uncoupler section has the magnet about a 1/32" above the outside rails.

 

Irontooth

[trains4fun]

I have a Fastrack setup with operating track and an uncoupler track.   I just got a good condition 1656, 0-4-0 steam loco from about 1948.  Everything is tight, not much wear.   When it runs across the uncoupler part of the track, the pickup rollers and of the engine body bottom out on the uncoupler magnet area.   It runs through the switches just fine.   It runs through the uncouplers when moving well but it bumps and wobbles when it hits the uncouplers.   

UPDATE...

Took my layout apart and did detailed inspection and found the center isn't dragging.   There is clearance between the center area and pickups with the uncoupler.   What is happening is the flanges on the drive wheels on the 1656 are deeper than the other engines and cars I have.   The Fastrack center area road bed rises up in the area of the uncoupler.  This causes the flanges to ride on the plastic vs the outside rails.  Just enough to cause the engine to wobble through the area.   This is what is different with the Fastrack vs. the older style track.