Pancho Villa

The film came about in a weird way because of the Hollywood blacklist of the McCarthy era.

Bernard Gordon had moved to Spain after being blacklisted and had worked with Martine before on 1972's Pancho Villa, which also starred Savalas.

Gordon said in an interview that the train set had been built for Pancho Villa, while Martine said it was acquired by a producer from 1971's Nicholas and Alexandra and wanted to make a film around it.

Whichever the case, Dussault and Zimet, who were also blacklisted from Hollywood, were brought on to write the script.

Despite the obvious similarities to John W. Campbell's Who Goes There, which became the source material for The Thing, Campbell was not credited, and there is a debate on whether it served as a direct inspiration.