A most unusual work shop in a tunnel formed when railroad trestles where buried in slag in North Bessemer to protect them from Nazi sabotage ?

Yes hard to believe but during WWII  many railroad trestles where buried in slag and such all the way up to the tracks to protect them from Nazi Saboteurs but we seem
We have a classic example near where I live  where a double set of trestles was buried and the Plum Creek Railroad which ran underneath  the trestles track section was made into a tunnel to allow it to keep operating  out to the coal mines in Renton.

Before trestle was buried

After trestle was buried

The twin trestles which where curved  can be seen covered in slag in above picture and where of vital importance during WWII as they belonged to  Andrew Carnegie's Bessemer & Lake Erie Railroad  which hauled Iron Ore down from ships which hauled it from iron ore mines in Minnesota  as well as limestone and coal and other commodity needed to make steel from many locations along the line as it snaked north to Erie.
Seeing how impossible it would be to post guards everywhere along these tracks which had many trestles along them  it was decided to bury them so Nazi saboteurs could not place explosives on them and knock them down which could take months to replace instead of a couple days when you just blew up tracks . While another explanation is the trestle where buried to strengthen the trestles to carry heavier loads but if this was the case why was the Allegheny  river bridge not reinforced   as well as other areas along route .  Approach Trestles where also buried about a mile up from these near where a magnificent steel bridge goes over the Allegheny river  in Harmarville as a way to help when the new bridge was built . and this bridge was guarded during WWII

https://books.google.com/books?id=-CoyAQAAMAAJ&pg=PA322&lpg=PA322&dq=north+bessemer+railroad+trestle+that+was+buried&source=bl&ots=LduCvuwT7H&sig=uYbk5SVUM1Ly5loGyl1TRFtJxf4&hl=en&sa=X&ei=0UjKVKqUMsWogwTZsIKoAw&ved=0CEIQ6AEwBg#v=onepage&q=north%20bessemer%20railroad%20trestle%20that%20was%20buried&f=false

Possibly the open sections under a concrete aqueduct which took the B&LE thru East Pittsburgh where also possibly filled in for same reason as where many smaller areas all along the route.

East Pittsburgh trestle you can see where openings where

http://pghbridges.com/articles/haer/blerr_PA508/blerr_HAER508.htm

Thou the Steel mill activity in Pittsburgh has greatly decreased the B&LE is still an active railroad now owned by Canadian National Rail

https://en.wikipedia.org/wiki/Bessemer_and_Lake_Erie_Railroad

However the Plum Creek branch stopped service service in early 60's  and the abandoned railroad tunnel has now become the workshop for an older gentleman who runs several small oil wells in the area. 

http://pghbridges.com/braddock/0602-4483/prr_plum_blerr_tun.htm

Some additional pictures of trestle and burying of it