As I think the original poster (
@365 Networker ) understood, the question is very dependent on class of traction. I can only comment on the units I sign.
On 377 units, the braking is described as "blended". If I select step 1 or step 2 or step 3 brake, I should get fixed levels of deceleration, regardless of speed and regardless of whether my kinetic energy is being converted into electricity or heat in resistors or heat in the brake pads. The system is pretty damn clever; it even uses the pressure in the air suspension to adjust the braking so that with a heavier passenger load the train makes a greater effort to stop.
As well as coping with the variations in passenger load, the system also has to deal with the variation in dynamic braking with speed. Effectively there is a fixed limit to the power that can be fed back to the 3rd rail or overhead, or to the resistors. The power generated by dynamic braking is proportional to the deceleration multiplied by the speed, so at high speed we hit the power limit and the dynamic braking is feeble. At low speed the motors are not being turned fast enough to act effectively as generators and so dynamic braking is feeble. At moderate speeds dynamic braking is useful. When I first apply the brake at high speed, the friction brakes self test and I see the pressure gauge go right up, then the dynamic braking kicks in and the friction brakes release their air and the gauge goes down a bit with the dynamic braking doing what it can and the friction brakes doing the rest of the work. As the speed declines and dynamic braking stops being power limited I see the pressure gauge drop slowly down to nearly zero as the dynamic braking takes on all the work. When I get down to around 10mph the motors become useless as generators and hence useless as brakes and the braking swaps back to friction, with the gauge going back up.
In theory, I should not be able to tell the difference between dynamic brakes and air brakes. If I have a unit with the dynamic braking cut out, I still use the same braking points. Whether I'm on a morning weekday commute (heavy heavy heavy) or a Sunday evening trip (light) I still use the same braking points. The only way to tell if you are being braked dynamically should to look at the brake pressure gauge.
In practice the system is not quite perfect, but it is pretty good. There are a couple of things that give away that the train has fallen back to friction brakes:
(1) brake discs don't wear evenly, so there is a vibration associated with friction braking whereas dynamic braking is smooooth.
(2) when the dynamic brakes cut out around 10mph there is a characteristic dip in braking for a moment as the friction brakes take over (which can be unpredictable by 1mph or so). If dynamic brakes are fully cut out there is no dip in braking and stopping exacty on the mark is easier!
On 377 units there is no circuit breaker in the cab (or other method) that allows the driver to choose dynamic vs. friction breaking. There is a method that fleet can use, but it is not available to the driver.
On 313 (Southern) units, dynamic braking is still available so far as I know, but the circuit breaker is always cut out and we are not allowed to reinstate it. I am told this is because the system was becoming unreliable. Having read old driver training material and accident reports (!!), I get the impression the implementation was somewhat cruder than on 377s and frankly I'm quite happy creating hot discs instead!
When driving a disc braked train in snow, the Rule Book requires drivers to do regular brake tests. The purpose of this is to break any ice that has formed on the mechanisms and to melt any ice that has formed on the discs and pads. So what happens on trains with blended brakes where the driver cannot choose to use the discs? On the 377s this is easily dealt with because the dynamic braking is not strong and for anything more than step 1 brakes the friction brakes are doing most of the work. However, I have read the driver training materials for the 700 units, and I see they have a "snow mode" which turns off the dynamic braking on those axles which have disc brakes.
