Plastic is a better insulator than metal. 
. The original plastic caps in 1967 ( cream colored ) were molded in a higher temperature plastic than today's black caps. I don't recall ever seeing a melted cream colored cap. Are the black caps even Lionel issued? My guess is cheap knock offs that use cheaper plastic.
And also, that turbine is going to draw more current than a MPC 4-6-4, running the smoke heater at a higher voltage.
