Please READ MORE for the video. Braking physics related with brake shoe friction ratio (K, L etc.) and vertical force of them (to the wheel bandage or disk or rail). Brake system ensure negative force when activate and rail-wheel contact force (adhesion) must resist to brake force. So, wheel rail contact force and furthermore, vertical force of vehicle is important parameter for braking.
Braking test regulation: EN13452
Some braking technologies convert the energy to electricity again via power system through braking force and resend to line to use on other trains which being in the same section or to burn in resistors.
f=m*a (f: force (N), m: mass (kg), a: acceleration (m/s2)) Also Jerk level should not exceed safety limitations. Emergency (harsh) braking creats negative (not completely negative, it is about vector) g-force. If the passenger about 75 kg. and deceleration force 3.0 m/s^2 and f=75*3=225 N
So, hold on tight the handles in railway vehicles.
Videos: Joó Ferenc Debrecen CAF Urbos tram brake testing
https://www.youtube.com/watch?v=O_9KBZM_e3M
widget787 ALSTOM Citadis Tram in Istanbul (with Emergency Braking)
https://www.youtube.com/watch?v=bgAumM1wHHg
Marius Ciocan Emergency brake tests D class tram - 60 km/h
https://www.youtube.com/watch?v=2Ark3JusZ8w
Khang Lu Train Emergency Brake Activation At 40 MPH
https://www.youtube.com/watch?v=ldF6tug8ykg
Tc Rail Heavy braking and sparks with cast iron brake blocks
https://www.youtube.com/watch?v=jm4LXmumR9s
TC Rail Brakes on fire
https://www.youtube.com/watch?v=LE5AMO83QRg