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:
- SIMBIDS (a subset of reversible)
All apart from the first are lines where trains can be signalled in both directions.
SIMplified BI-Directional (SIMBIDS) is a poorer-man's 'reversible'. It has no AWS magnets for the signals and the permanent AWS magnets for the normal direction signals are not suppressed, so trains receive lots of unnecessary horns ("Special AWS working"). It 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.
Reversible used to have the same usage restrictions as SIMBIDS, until Spring 2015 when it became allowed to use it for regulating purposes as well. 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).
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 reversible and bi-directional lines with a 'normal' direction and without.
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, and this was approved for use for regulating long before the rest of the WR reversible was. Only the up line is fitted, so as trains can't be signalled to pass each other in opposite directions, it 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 was, of course, bi-directional.
There were one or two relaxations to the reversible limitations over the years before it became generally allowed to be used for regulating. For example, down trains terminating at Swindon was permitted to arrive in platform 4 and start back towards London via the Down Main Reversible to Bourton from about 2007, and down trains for Swindon Yard were allowed to cross to the Up Main at South Marston and enter the yard at the Highworth end from about 2010, but there are some limitations on that. These restrictions have been nullified now by the general allowance of reversible working whenever required.
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.