Fastrac back to back switch stopping engines engines

Check where the pickup rollers are when it stops. They are probably each on the isolated sections of the switches. I have had to insert a short track section to get one pickup on a hot piece of track. A picture might help.

I could make some guesses why the trains are stopping, and it's likely more than one issue.  Top suspects are folded tab electrical connections inside FasTrack switches not making good contact and track connecting pins loose inside the rails.  Sometimes checking for voltage with a meter won't reveal where the fault lies if there's a high resistance connection.  The high impedance of the meter won't load the circuit enough to drop the voltage like a motor or incandescent bulb does.

I suggest checking either with a 12V incandescent lighted test probe, like for auto electrical system testing or using a incandescent lighted car with a single pick-up roller to find the exact locations where the electrical faults lie.  Once you do that, there are lots of topics here on the forum detailing how to correct loose track pins and folded tab jumper connections inside FasTrack Switches.

Classic problem with back-to-back switches.  This is the reason that many new locomotives have more rollers, that helps eliminate dead spots.  For Ross or Gargraves switches, it's not unusual to have to add a relay to switch power to some dead rails based on the switch position to minimize dead spots.

Note that these two rails are dead rails.  However, for some switch types, it's possible to connect to these to a relay switch to power them selectively to minimize dead spots.

Ok, now what? I would have thought my Legacy Niagara would have enough roller connection. Do I need to remove a switch which will mess with my track design or try to insert a short track in-between the two as suggested by John?

Well, typically the more spacing between switches you can give, the better things will work.

One issue with a short track between switches.  That may just cause the dead spot to show up for a different locomotive with wider roller spacing.

Case in point: I have two switches that have a short 2" spacer to make the track plan work.  That caused a rather long K-Line E8 diesel to stall.  Turns out that the roller spacing on the E8 is actually quite a distance, just the amount needed to find the dead spots for each roller over that specific switch configuration!  The K-Line E8, being an older locomotive, doesn't have the dual roller assemblies that many newer diesels have, just one on each truck.

On more than one Lionel steam locomotive, especially shorter steam with the IR drawbar, I've added a single wire tether to share the locomotive and tender pickups, that normally solves any switch transition issues.

Basic question but sometimes the obvious is what we miss. Are you sure that the wires under the short pieces with the break in the center rail are in place?

Mikki