Over Bridges

Bridges, level crossings and stations require special mast for differing clearance reasons.

Bridges present their own problems. When overhead systems are applied to a line many of the bridges have to be rebuilt to clear the wires. Even with this rebuilding whether the bridge is raised, or track bed lowered often the catenery wire has to be lowered to give adequate electrical clearance between the structure and the live wire. The minimum clearance is now 270mm. This is the “static” clearance; the upsweep caused by the pantograph reduces this to a “passing” clearance of 200mm. Under special circumstances this minimum static can be reduced to 200mm with permission of the local electrical engineer or even 150mm by the Department of Transport. The contact wire height can be reduced to 4.19m under bridges. To enable a clearance of 270mm under a bridge the catenery wire is lowered by reducing the encumbrance as well as lowering contact wire height if need be. At some locations it is necessary to splice a section of contact wire into the catenery wire and run a section of double contact wire under a bridge, as there is not the room for normal arrangement of the catenery and the contact wire.

There has always been a spilt between who is responsible for what on the railways. The electrical engineer is responsible for his mast and maintaining the electrical clearance from the civil engineers structures. Some times to do this the electrical engineer has to fix special registration arms to the underside of the bridge, this is done with the permission of the civil engineer. These are made from glass fibre and providing their own insulation.

Some Mk1 wire supports under bridges have standard insulators.

Some locations where Mk1 equipment is used it is common to find the catenery wire being supported on a mast but the contact wire does not have a registration arm. I have not found out the reason for this additional support for the catenery wire.

On multiple tracks both portal and headspan have reduced encumbrance when adjacent to a bridge.

While on the subject of bridges and overhead equipment, as I have already stated sometimes the track bed is lowered, this can be very noticeable. Yet I have never seen this modelled.

 On the approaches to Liverpool Street station in London there are some mast with varying contact wire heights as well as reduced encumbrance, this is due to the tracks passing under an arched bridge.