The momentum of the girl and trunk system was zero before she jumped off the trunk. When she jumps off the trunk, she exerts a backward force on the trunk and the trunk exerts a forward force on the girl (Newton’s third law).

The girl moves forward but the trunk does not recoil. The reason is that there is a force of static friction in between the trunk surface and the railway platform. The force exerted by the girl was not enough to overcome this friction and so the trunk does not recoil.

Since, there is an external friction force on the system in this case, so linear momentum is not conserved here.