Waiting for the next train

This woman decides to buy a new cupboard that you have to assemble yourself. Back home she reads the instructions carefully and assembles the cupboard in the bedroom.
It looks really neat. Then, a train passes and the whole cupboard collapses.

Thinking that she must have done something wrong she rereads the instructions and reassembles the cupboard. Then, a train passes and the cupboard collapses again. Now, fed up she calls customer service. She is told that this is quite impossible and that they'll send along a technician to have a look.

The technician arrives and assembles the cupboard. Then, a train passes and the cupboard collapses. Completely baffled by this unexpected event, the technician decides to reassemble the cupboard and get inside it to see whether he can find out what causes the cupboard to collapse.

At this point, the woman's husband comes home, sees the cupboard and says: "That's a nice looking cupboard", and opens it. Says the technician: "You may find this hard to believe, but I'm just standing here waiting for the next train."