Do you have a milling attachment for your lathe? Hold a chucking reamer in a 3 jaw chuck and mount the rail on the carriage. Then use the milling attachment to center the hole on the reamer, and use a slow feed on the carriage to ream the hole. That's how custom rifle barrel makers gun-drill their barrel blanks before rifling them. You can flow water soluble cutting oil into the drilled hole in the rail to keep the reamer lubed. If the shank on the reamer is too short, weld an extension onto it.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!