a couple of years ago I asked a question of the sites followers. Which GP 40-2 unit had the roof numbers in the reverse direction. No one knew or could remember. It wasn't mentioned in the green machines book either. I finally found enough info on an old vhs tape of the "reading Co. Scrapbook vol.3 Willmington & Nothern just at the end of the tape. Finally a roof shot of 3 units shows 3675, & 3674, the bottom of the roof numbers face the short hood and 3671 the bottom of the roof numbers faces the long hood. So the answer is 3671 is backwards from the other 4 units. And by the way the numbers on the Atlas units are too large the closer size decals can be found on old herald king green switcher decal set the cab decals in yellow. I think a scale 18". There is also a cab roof shot of 3675 in "Reading Diesels Vol 2 " a Dale Woodland book, on page 145. You can scale the number and compare it to the length of the cab roof. Happy 2019! There! I just gave you fellow rivit counters something to do this year.

What was the reason for putting the numbers on top of the cabs anyway?

Identify the units from the yard towers. Well into the day and age of radio communications, there must have been additional factors that drove the addition of cab roof numbers? They had to be nearly directly in front of the observer to be readable.

Some speak of the idea of assisting Philadelphia and PSP helicopters to be able to identify trains when interacting with RR Police or just general police work. If that has any substance why didn't there seem to be any effort to retrofit numbers on other green & yellow units that weren't scheduled for full paint jobs.

Just curious.
Rob