Slab track in the M&SJWR

Metropolitan? & St Johns Wood Railway (I guess - it did take a while for me to think it up)

BS95?

I must admit I don't recall seeing any of this flat bottom rail you speak of. According to the current list of line closures and there is no BTR due in that area during 2007/08. In that time track renewal is to be done on the Circle, parts of the H&C and north of Harrow on the Hill.

Balfour Beatty Rail have the contract to carry out the BTRs for Metronet on, I believe, a design and build contract. I'm not in the know with BBR but I would be surprised if slab track is a solution they would use. The idea of slab track was bandied about at a time when Metronet were getting stung for a high number of signal failures in that area. The belief was that water ingress through the tunnels and the state of the track ballast was the cause. A slab track formation would have a designed drainage system and save on maintenance costs. I imagine the outcome of any cost analysis was that the financial outlay would only be paid off in maintenance savings long after the expiry of the PPP contract.

Not only would you need a long term line closure, the construction risks would also be high and there is the problem of single line working where clearance is restricted and prevents use of any heavy rail plant. It is unlikely the Class 66 Loco (or NETF) and its 400m long multi functional track laying system with slinger trains will be employed south of Preston Road. Drainage replacement work on the District has used road railers to dig out the six foot but that was in twin track tunnel. It may be possible to use mini diggers to excavate the existing track bed but track work would probably be labour intensive and be done with light equipment like ironmen and track trolleys. I've seen track laying equipment used for single line working before but there still is the issue of adequate clearance.

There is also the structural integrity of the tunnels to consider, the surrounding soil may not be able to carry the additional dead weight of a concrete slab without subsiding. In buildings it would be normal practice to install a raft foundation to spread the load or use piled foundations but it would be expensive to do it here.

For much of the line the tunnels are a single track circular (ogee-like in some areas) brick arch type with a shallow invert. The structure gauge is approx 14 foot diameter and rail level is 2 foot above the invert. The drainage is jointed vitreous clay pipe and at the time not much was known about its condition. Being so close to the underside of the sleeper and being a brittle material the pipes would crack under the vibration. The drainage was intended to be a carrier drain whereas protection against seepage was the tunnels outer lining which has failed in places after over 100 years. The track ballast has been highly contaminated with fines and consolidated under track load over a long period of time into a solid concrete like material. The result is that any water seepage through the tunnels will sit on the track as it cannot drain away and the ballast no longer absorbs train loading so the track will take a hammering.

I believe the tunnels are of newer but similar construction north of Swiss Cottage Disused as the line was diverted for the Bakerloo tube tunnels to join Finchley Road. The former tunnels were infilled with weak concrete and the deep tube tunnel were bored through.