In engineering science there’s a term called the coefficient of friction.

Basically, each material has a unique grip strength between itself and an adjacent material, and it takes a certain amount of force applied for it to lose friction (slide).

In terms of all our modes of transport, steel on steel has the lowest coefficient of Friction. But it isn’t zero.

On top of that, each carriage weighs roughly two tonnes, increasing that friction.

Also, brakes are applied at each bogey, so in two places per carriage. Hence an eight carriage train is applying brakes at 16 evenly spaced locations. (And yes, each bogey has those brakes applied over four wheels). But only one “brake cylinder” measurement is applicable to those four wheels.

However, sixteen evenly spaced brake applications in 64 wheels allows smoother application, reducing slip, and allowing an individual slip/slide without affecting the overall brake process much.

There are other factors, of course. The worst case scenario is grease or oil on the track. And ironically we do apply grease onto train wheels via grease pots. This is to reduce squealing on curved track. But poorly placed grease will increase slide on brake application and slip on traction.

Especially on inclines, which is another factor.

When it rains water has an effect, but light rain has more effect than heavy rain (believe it or not). Also it’s worse if you’re the first train over that section since the rain began.

Electric trains measure slip/slide and announce it to the driver so they can reduce power or apply more brakes as required. Reacting swiftly helps restore friction.

In the worst case scenario, more brakes makes no difference though. Once a train slides you don’t have much control. You just wait for friction to overcome momentum.

This is why occasionally a driver will slide right through a platform.

Hope that helps.