The rule book (TW1 Section 4.6) states that:
You must test that the automatic brake is working properly by carrying out a running brake test.
When you carry out a running brake test, you must do so from a speed that is high enough for you to be sure that:
the brake is operating effectively
the speed of the train is being reduced.
When working multiple-unit trains you must carry out the running brake test as shown in your train operating company instructions.
These are SWR-specific instructions - if you work for a different TOC then these will not apply.
You must carry out a running brake test at the first practicable opportunity after starting a journey for each new driving cab you are driving from.
When completing a running brake test you must take into account signalling and prevalent weather conditions and conduct this ideally no later than the first stopping point.
When a running brake test is made in these circumstances, you must make sure that the brake is working properly and a reduction of speed is achieved.
Additional running brake tests should be carried out as follows:
when approaching a steep falling gradient during freezing weather conditions,
on the approach to a terminus or dead-end bay, if the brake has not been used for a long period of time,
whilst travelling down a falling gradient,
changes in the weather including initial rainfall or constant light drizzle,
generally throughout the low adhesion season,
at locations of known low adhesion based on previous knowledge and experience,
prior to entering high-risk sites during the autumn season.
A running brake test is not required if you have already completed one from the cab you have previously driven from and you have remained with the same train throughout.
When snow is falling or fallen snow is being disturbed by the passage of trains, a full service brake application must be made every three to five minutes and a reduction of speed of at least 10mph achieved. The train speed must be restricted to 10mph below the permissible speed for the train concerned over each portion of the line. The speed need not be restricted to less than 50mph.