Originally Posted by AlanRail:
I see that a engine's knuckle coupler was not strong enough in the space allowed to pull the train so they used a drawbar.
No! Knuckle couplers were/are DEFINITELY strong enough to pull the whole train, i.e. the rear of every tender had/has a knuckle coupler, which PULLS THE WHOLE TRAIN.
I simply don't know how many times I have to explain that THERE COULD NOT BE ANY SLACK between the engine and tender! The the Steam engine's reciprocating drive design, if the tender was NOT "drawn-up-tight" to the engine's buffer plate, there would be TERRIFIC crashing & banging between the engine & tender. That is why there was a spring loaded radial buffer assembly in the front pocket of the tender. That spring loaded assembly applies thousands of pounds of force to keep the tender TIGHT AGAINST the radial buffer, so that there is ABSOLUTELY NO SLACK or loose motion between the engine & tender.
You'd think that the Penn would have had the common decency to use a tight-lock coupler on that big, hog-molly Q2 ! ... Anyway, UP did use tight-locks on the big 8500GTEL's between the A and B units, as well as the tenders. Not the same stresses, for sure...but can you imagine what would happen if the A and B units were to become uncoupled under load? Those are NOT just MU cables running between the units, but full power transmission lines from the generators. ZZZZZAAAAP !!!!
It's fairy easy to change a knuckle . Can you imagine what would be ripped apart if the engine and tender separated. Air hoses. water lines, steam lines. Maybe even a crew member. Yes HW no slack. we get it.
I once heard that the difference between a steam engine and diesel was... it was easy to find the problem on a steam engine but hard to fix.... Hard to find the problem on a diesel but easy to fix.
SIMPLE ANSWER: The limits of close-coupling are the track radii/diameter.
