。
She made room on the train for an elderly woman.
Literal
She [topic-は] train [possessive-の] inside [at-で] elderly-woman [for-のために] seat [object-を] squeezed-together.
席を詰める means 'to squeeze together to make room' — 詰める ('to pack / stuff') applied to seating. This is a practical expression for public transit etiquette. 老婦人 ('elderly woman') is a formal, respectful term — in conversation, おばあさん or お年寄り would be more common. The sentence captures a small act of consideration common on Japanese trains.