“Train d’enfer” is a French expression that is a little difficult to translate into English. You might think it has something to do with railways; but actually and nowadays, it is more often used to describe a kind of relentless momentum and dynamic movement.
For example, you could describe a midfield diamond formation in football as imposing this kind of rhythm on a game. In cycling a breakaway effort, leading from the front, might also be described in these terms.
But the expression is obviously linked to railways and steam locomotives. I don’t think the expression has anything to do with runaway trains. It’s more to do with the huge efforts of steam powered speed records.
Don’t forget that, in the big steam express locomotives of the mid 20C, the engine was powered by coal. The coal had to be endlessly shovelled from tender to fire-box. This was exhausting work.
I think the expression comes from the combination of mechanical speed and human effort. The association with hell fires comes from the glow of the furnace reflected in the frantic movements of the fireman.
The poster, above, by the Irish artist, William Orpen, gives a very good impression of this dramatic scene.
The expression is so dramatic and meaningful that it has been used several times a film title.