How do railway companies replace bridges/tunnels without disrupting service?

[zkr123]

For example, Metro North is going to replace the swing bridge in South Norwalk, CT. How do they put a new bridge in place without disrupting the busiest rail line in the US?

[MidlandMike]

When Amtrak replaced the Niantic, CT bridge, they built the new bridge beside the existing bridge and then rerouted over it.  I suspect if a bridge's abutments are still good, they may build the new bridge near the site, and then switch it in during a construction window

Tunnels are sometimes paralleled, and sometimes enlarged in place during construction windows.

[Overmod]

zkr123

How do they put a new bridge in place without disrupting the busiest rail line in the US?

Part of an answer is in the recent thread on the "R crane."  Another can be seen in some of K.P.Harrier's posts on double-tracking the Sunset route, with the bridges built parallel, track 'shooflyed' to them, new abutments constructed in the right alignment and the spans transferred to them, and the "brand new" supports built for the parallel bridges then demolished (!)

For tunnelling, I can think of no good approach other than periodic interruption of traffic while some increment of work is done, or diversion of traffic over some parallel route while a more extensive job is done.  The method being used for the ex-PRR North River tunnels -- part of what I would consider one of the most heavily trafficked segments of line in the country -- is instructive.

[pajrr]

Back in the late 60's- early 70's the Erie Lackawanna replaced a thru-plate girder bridge over a highway. The new bridge was built alongside the old one. After a Friday evening rush hour was over, the railroad was closed and the new bridge literally slid into place as the old one was slid out of the way. The railroad reopened at Monday morning rush hour, just like nothing had happened.

[oltmannd]

Here's a timelapse of a bridge replacement.  Torrence Ave in Chicago

http://youtu.be/ByCin67MPuM

[tomikawaTT]

In days past, a new bridge would be built next to the one being replaced, the tracks would be re-aligned to the new bridge and the old bridge would be dismantled, leaving masonry piers and abutments to erode away.  A satellite view of Harper's Ferry, WV, is a good example.

Major tunnel reconstruction/modification might include building a new tunnel first, then rerouting all the traffic through it while the old tunnel is opened up to modern clearances.

On the Tehachapi Pass route, the old tunnels are being paralleled by new cuts.  Amazing what modern earthmovers can do that was beyond the capabilities of people with hand drills, black powder and mule-drawn carts.

Chuck