Here’s the best photo I could find of an RS-3’s fuel tank. I can’t quite figure out what the thing is at the bottom of the long tank, that looks like it has an X on it:
a #10 phillips head screw? 
… Sorry Ray I couldn’t resist. Hopefully someone will have the answer and we’ll both learn
Just a guess on my part, but I’m thinking it’s a “jacking” port. Minor derailments are often fixed with jacks.
Ralph
I don’t think they would have a jacking point on the fuel cell. However in the car business I have seen guys do it with not so good results.
I’m gonna take a guess myself and say that it’s possibly a drain but the picture is too grainy?
This album has a bunch of great close-up RS3 photos
http://s291.photobucket.com/albums/ll292/12bridge/Detail%20Shots/RS3/
I don’t see that port on this one. My vote is a drain?
Thats where they hook up the turbo encapsulator , tht prolly an older RS3 from the transition days they’d tie the diesel to steam engine so it would be ready to go the next morning…
Bob McCown said:
This album has a bunch of great close-up RS3 photoshttp://s291.photobucket.com/albums/ll292/12bridge/Detail%20Shots/RS3/
Great find! Wish I’d seen some of those photos earlier. I’ve looked at over 2000 pics of RS-3s, and it’s hard to find good, clear close up shots.
… and I’ve been kicking myself daily for not shooting some myself when I was in Ely back in August.
David Russell said:You're probably correct about the drain. Of course they wouldn't mount a jacking point on the fuel tank. I was thinking under the fuel tank, mounted to the frame. Ralph
I don't think they would have a jacking point on the fuel cell. However in the car business I have seen guys do it with not so good results. :) I'm gonna take a guess myself and say that it's possibly a drain but the picture is too grainy?
In the colder climates, it may be a steam port for a steam coil in the fuel tank to assist in keeping the diesel fuel from gelling. I am mostly a steam guy, but that would be my best guess.
Bob C.