Do the 1121 switch have the auto derailling function? I would like to do double switch back and be able to run the large fange wheel Marx’s engines.
No, they do not, however the 1122 does.
You can control 1121s with an insulated track section, but when I tried to use them with fat-wheel Marx engines I was disappointed. The engines still bounce on one leg of the switch when they go in one particular direction. The bounce isn’t enough to cause a derailment every time, but it happens enough to be irritating. I did a Christmas layout one year that was two double-reverse loops using Lionel 1121s, and if I ran a Marx locomotive on it, it would always run into a problem within about 10 minutes. I scrapped the layout and sold the 1121s.