Reversible Signalling

As with a lot of nomenclature, there is a lot of regional variation in naming, application, useage, etc. The following description relates to Western Region thinking:

There are four ways of classifying running lines in terms of their direction:

• One-directional
• Bi-directional
• Reversible
• SIMBIDS (a subset of reversible)

All apart from the first are lines where trains can be signalled in both directions. The only* difference between them is procedural: Reversible (including SIMBIDS) lines can only be used when the correct line is not available, due to engineering work, infrastructure failure, train failure, etc. It can't be used for commercial/performance gain. Bi-directional can be used in either direction whenever required for any purpose.

To break a common misconception, the difference is NOT to do with there being a 'normal' direction. There are many bi-directional lines with a 'normal' direction and without.

SIMplified BI-Directional (SIMBIDS) is a poorer-man's 'reversible', it can only be used in the same circumstances* as reversible, but it has no AWS magnets for the signals and the AWS magnets for the normal direction signals are not suppressed, so trains receive lots of unnecessary horns ("Special AWS working").

Almost all the Main and Badminton Lines on Swindon Panel are reversible.

Reversible exists in the following sections: Milton - Wantage Road/Challow - Uffington - Bourton - South Marston* - Swindon East* - Swindon West* - Wootton Bassett (Main) - Thingley Junction - Bathampton Junction - Bath West - North Somerset, also Swindon West* to Wootton Bassett West - Hullavington - Chipping Sodbury.

There is also reversible between Teighmouth and Dawlish Warren, however, this has been approved for use for regulating, so is really 'bi-directional' now, according to the long-standing definition. This, and several other lines, are locally referred to as 'reversible' when they are really bi-directional (such at Exeter SD to Central and Yate to Westerleigh).

West of Thingley Junction there is SIMBIDS onto the Bristol Panel area.

The lines through the platforms at Swindon are bi-directional. There is also a short section of bi-directional on the Down Main between Thingley Crossovers and Thingley Junction.

The Gloucester Line is not anything, except for the single line part which is, of course, bi-directional.

There has been one or two relaxations to the reversible limitations over the years, for example, a down train terminating at Swindon is permitted to use platform 4 and start back towards London via the Down Main Reversible to Bourton, but there are some limitations on even that.

The locations with stars, above, have limited flexibility to cross trains over.

South Marston only has a facing crossover. Swindon East only has a facing crossover. Swindon West only has a trailing crossover.

• - Updated Spring 2015 - reversible (but not SIMBIDS) can now be used for traffic purposes. The only remaining restriction on 'reversible' stopping it merging completely with 'bi-directional' is that two trains can't pass on reversible in opposite directions (ie, both on the wrong line).