The original cords on Lionel transformers had a heavy duty insulation. I have a Type R that I've had for 70+ years, and the insulation has no cracks (probably should be replaced anyway). I have several ZWs & a KW that I've had for 50 years, and the cords have no cracks. While 2-conductor, they are not the flimsy lamp cord you can buy for rewiring lamps.
Your sarcastic comment about the original cords is incorrect. The issue is not insulating 120-volts--it's being durable to withstand handling.
dpg: I'm not so sure that a replacement internal breaker would be a sound solution. It only breaks the U circuit, leaving you unprotected against shorts between any two of the 4 hot outlets, which can occur easily Consider using external breakers on all 4 hot outputs, and feeding all ground return to through a single 10-amp breaker, tied to several of the U posts since the rivets holding the posts to the internal bus bar do lose good contact.