A railroad car with mass m is moving with an initial velocity v when it collides and connects with a second railroad car with a mass of 3m, which is initially at rest. How do the speed and momentum of the connected car system compare with those of the car with mass m before the collision

Same momentum but speed is reduced 4 times.

Explanation:

According to the law of momentum conservation, the total momentum of the system before and after the collision is the same

Before the collision, the bigger car is at rest, only the 1st car of mass m is moving at speed v

p = mv

After the collision, both cars are connected and moving at speed V

P = (m + 3m) V = 4mV

These 2 momentum are equal

p = P

mv = 4mV

V = v/4

So after the collision, they have the same momentum but the speed decreased 4 times