Dad here:

Sometimes you learn things you never expected. Here is how you get train milk:

When a steam train has too much steam it goes into a tube

where it becomes milk.

Chocolate milk.

If you wait for it at a crossing, with a cup, you can fill your cup with cold chocolate train milk at the crossing.


So, if you're in need of cold chocolate train milk (if, for example, your cup doesn't have enough) then just sit down at your nearest train crossing and wait for the next steam train. I can tell you from experience it won't be long coming.