Slab track in the M&SJWR

[Deleted]

stanmorek said:

Finchley Rd to Baker St IMHO is the most difficult section of the SSR to do as a good part of it runs in single tunnels with inverts and track slab was considered as an option at one point.

Having now seen the growing presence of BS113 flatbottom rail lurking amongst the four-foot and six-foot on the M&SJWR, does anyone know if/when track replacement will begin along this stretch, and whether or not slab track may be used?

At least it would be relatively painless to do - if the BTR folks start at Baker Street station throat and work their way north, Met services can do the mainline shunt at Swiss Cottage and collect the hordes from the Jubilee Line trains (which run 2/3 full north of Baker Street anyway, so lots of space IMO), thus negating the need for a rail replacement bus service during the shutdowns.

I am curious though as to what kinds of extra work may be needed, particularly with regards to any potential gauging issues for A stock or S stock after the new rails are in place. I am also curious as to whether or not Metronet has managed to regain gain enough 'institutional' knowledge to attempt an NR-style blockade to get everything done at once.

[Chris M]

For the uninitiated, please could you expand the "M&SJWR" acronym.

[c5]

awkward42 said:

For the uninitiated, please could you expand the "M&SJWR" acronym.

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

[Deleted]

so if flatbottom is BS113 wot is bullhead?

[Deleted]

aetearlscourt said:

so if flatbottom is BS113 wot is bullhead?

BS95?

[stanmorek]

aetearlscourt said:

so if flatbottom is BS113 wot is bullhead?

RBH 95 lbs/yd.

[stanmorek]

theonekea said:

Having now seen the growing presence of BS113 flatbottom rail lurking amongst the four-foot and six-foot on the M&SJWR, does anyone know if/when track replacement will begin along this stretch, and whether or not slab track may be used?

At least it would be relatively painless to do - if the BTR folks start at Baker Street station throat and work their way north, Met services can do the mainline shunt at Swiss Cottage and collect the hordes from the Jubilee Line trains (which run 2/3 full north of Baker Street anyway, so lots of space IMO), thus negating the need for a rail replacement bus service during the shutdowns.

I am curious though as to what kinds of extra work may be needed, particularly with regards to any potential gauging issues for A stock or S stock after the new rails are in place. I am also curious as to whether or not Metronet has managed to regain gain enough 'institutional' knowledge to attempt an NR-style blockade to get everything done at once.

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.

[stanmorek]

I passed through the area the other day and did notice some pieces of flat bottom rail in the covered way section where the junctionwork on the northern approaches to platforms 1-4 are. They are likely to be spares for that section as it is flat bottom rail until the headwall where the NB and SB diverge into single running tunnels.

Another thing about slab track, I remember not too long ago when the Jubilee had a series of mysterious signal failures in the Green Park area. The cause was eventually found to be steel reinforment bars in the slab were in electrical contact with the track circuits thus showing up as "ghost trains".