The original Union Tunnel did not anticipate electric trains so did
not have sufficient clearance for two tracks with catenary. After the
tunnel's floor was lowered, one centered powered track was installed.
Electrification of the tunnel had been discussed for decades as a way
to reduce smoke from trains. The Baltimore Sun of May 11, 1909 describes
discussions of a new city ordinance to require electrification, with
the PRR reporting it would be too expensive.
Adjacent the tunnel is this disused, and mostly pillaged, utility box,
with a few bits of remaining equipment like these General Electric
undervoltage relays that appear to date from the 1930s.
From here to Washington the PRR employed Oilostatic Okonite Cable for
its 132 kV transmission lines, that choice driven by the special needs
of running power lines through the Union and B&P Tunnels. Between
2004 and 2009 these original cables were replaced since they had
outlived their expected service life.
replacement project report (PDF)