The song's lyrics are similar in the chorus to "El Tren Blindado," or "The Armored Train." That may have been the origin.

As for the chorus:

Venga jaleo, jaleo
- different interpretations mean different things. In "Tren," this line is an onomatopoeia for the whistle that a train makes. However, it can be used to refer to a general din (a "jaleo").
Sueno de una metralladora
- literally, it means "sounds of a machine gun." In "Tren," an image is drawn of a train armed with a machine gun firing indiscriminately into some fields. However, in the other version, it can represent any skirmish between the Brigades and the Nacionalistas.
Y Franco [or Mola, etc.] se va a paseo
Y Franco se va a paseo
- as the song plays out, different names of fascist generals are substituted for "Franco." It means that he takes a walk or leaves, and could be used to indicate walking smartly, jogging, or even sprinting. Some say that it indicates that he is running away from the train or the battle, and others say it is symbolic of his imminent defeat.

This is a fairly subjective song, and the translation depends on the listener and the context. Hope this helps.