When Mr. Beechcroft boards the subway, he sits down on the second cushion and starts to lie down. In the next shot he's well into the third cushion as he lies down fully.
After Mr. Beechcroft eliminates all other people, he is still transported by subway even though no one exists to operate it. However, he's making the subway move using the same power of concentration that allowed him to eliminate humans.
When the elevator doors open in the lobby of Mr. Beechcroft's office building, there is no edging/gap/space between the floor and the elevator shaft, revealing that both the elevator and the lobby are free-standing sets built on the same floor.
(at around 4 mins) In the upper left corner, a boom mic shadow can be seen.