The last post by 'The Planner' is the definitive position on the old Western Region. The decision to run OEO is also based on whether any power notches have been isolated on the 'good' power car plus any other factors such as known overheating problems and high ambient temperatures. Railhead conditions also have to be factored in.
The rear power car powering is better because up to 36 wheelsets will have cleaned the railheads ahead of it and the effect of pushing forces the power car to dig in for better adhesion. This is particularly so during times of poor railhead conditions e.g. damp mornings in the leaf fall season.
If the dead power car is completely dead, the good power car has to sacrifice about 250 hp for auxiliary train supply throughout the set. If the dead power car can have it's engine running but not supply traction power, it can normally be used to provide auxiliary power supply allowing the good power car to use all its power for traction purposes.
The reason for all this is because one power car does not have sufficient power to pull away on steep gradients and will stall, potentially causing damage to traction motors. However, it does have the power to maintain forward motion if it hits the bottom of most gradients on the run, hence the Sectional Appendix states that a clear run must be guaranteed. If the train comes to a stand while running under these conditions, the driver is not allowed to attempt a restart but must call for assistance. Certain gradients e.g. St David's Bank are considered too severe for OEO running because of the excessive gradient, sharp curvature and restricted approach speed.
Back in Valenta days, 110 mph was achievable with ease given a sufficient clear run on Brunel's billiard table.
