Brian,
If you're talking about the "12", "25", and "40", those are row counters (though I'm not sure what the technical term is).
If you look at German sheets of that period, some had numbers across the columns showing the cumulative value, and/or down the rows showing the value of a single column. For example, here's a Generalgovernment sheet of 50+50 Groschen stamps with both:

Since three of your trains are from the bottom-left corner, the numbers match the value of the stamp.