Sir Keir delivered a statement to announce his planned departure from Downing Street.
The Labour leader says he will remain prime minister until his successor is chosen.
A passenger praises a member of railway staff who, despite being in pain, helped the injured.
Staff member who raised concerns about nursery worker Nathan Bennett was "ignored" for months.
Jamal Coombes, 17, was found fatally injured in Battersea on Saturday.