If a running metro stops abruptly, passengers tend to lean forward. This is due to:

• According to the principle of inertia of motion, an object tries to remain in its state of uniform motion unless acted upon by an external unbalanced force.
• If a moving metro stops abruptly, passengers tend to lean forward because the lower part of their body comes to rest on the ground along with the metro while the upper part continues to move due to the inertia of motion 
• As a result, passengers lean forward.

Therefore, we can say that when a running metro stops abruptly, passengers tend to lean forward due to the inertia of motion.