BaltACD, knows what he's talkin' 'bout.
I don't know whether it's weeks or months of my 42 years, in engine service that I've been stopped while the maintainers tweaked the drawbridge and its signal circuits to allow me to cross the span.
Where I had rights to run included 10 moveable (draw-) bridges to cross. All of them got me....uh, OK, one didn't.
Yes, I'm part of the roster which put two engines, 5 locomotives, into the Napa River at the open drawbridges.
Something else: turntables are moveable bridges....right?
Exact alighnment? Santa Clara and Watsonville Jct's turntables required balancing the locomotive on the 'tables, so that the 'table's engine, compressed air from the engine's main reservoir powered it, wasn't stalled or didn't spin its drive wheels.
When your engine entered that 'bridge it climbed or descended a gap which permitted attaining the balance point. I doubt inches, but less than a couple of lateral feet. Try that with an FM Trainmaster, that was a big locomotive then.
Highway bridges only have to be in alignment on a single plane - the plane of the roadway and highway vehicles can easily withstand minor alignment errors.
Rail bridges have to be in EXACT alignment on all rail connections, both virtically and horizontally and locked in those positions for all the safety mechanisms to permit lining signals across the bridge. Railroad operation does not permit the operation over the span with mismatches that would be allowable in highway use.
Never too old to have a happy childhood!
Except for the catenary sled and items related to the overhead, most of the same features apply to movable bridges anywhere. Infrequent movements may not be an issue in failures as the movable street bridges on the South Branch of the Chicago River work rather smoothly despite being raised only several times a year.
Railway age article on how moveable bridges work. This article applies to all RRs but posting in passenger forum as the higher speeds and electrification causes additioonal complications. After reading is it any wonder that Amtrak wants to replace all? As well MNRR? Wonder if the Portal bridge failures are a good example. Those bridges that are infrequently used may be more prone to failure such as the PATH bridge section of Dock Bridge last month?
http://www.railwayage.com/index.php/m_and_w/movable-bridge-best-practices.html?channel=5
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