Riding on a Chiltern Railways British Rail Class 168 DMU Bombardier Turbostar 

This was a train service from Harrow on the Hill to Rickmansworth.
These trains use hydrokinetic transmission, which works like the torque converters in automatic transmission cars.
Chiltern Railways is one of few railway operators serving London that still use Diesel trains. These units have been around since the late 1990s and have undergone rounds of refurbishment.
Taking this from London to stations towards Amersham is much quicker than the Metropolitan Line due to the fewer number of stops.