Potomac Yard clearance limitation, or was it the Baltimore tunnels?

[nw-mailing-list at nwhs.org]

As mentioned by Jim Foley, the controlling structure on the  Washington
Division (now NS's Piedmont Division) was at MP 112.2, in  Charlottesville.  
This
is an overhead bridge that carries U.S. 250 (Main Street) across the  tracks.
Because the structure is an ARCH, not all tracks would  clear TTX cars 
exceeding
17'9".  NO fully enclosed auto racks would clear.  Clearance was  so critical 
that
Southern had installed a detector at Proffit, six miles or so north of the  
structure.
(I don't know where the detector was for movements approaching from the  
south).
As a train approached the detector, it would project a beam across the main  
line.
If an excessive dimension car broke the beam, it would set into motion  
measures
to stop the train before it passed beneath the Main Street arch.
 
Clearance on the St. Thomas Division was so critical that auto racks  moving
Windsor-Fort Erie were equipped with 33" wheels, not 36" wheels.  GM  had an
assembly plant at Talbottville, Ontario which required such  equipment.
                                                                  Harry Bundy
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