A US term for a small power and brake controller provided for the driver at certain positions on a locomotive or multiple unit train for shunting movements where the expense of a driver's cab is not considered necessary. Also known in the UK as a shunting controller. The term is derived from the US name for a person working in an engine shed under the operating foreman. This is in turn derived from 'Ostler' who looked after the horses for the mail coaches, so it's a survival from English practice.