The passenger service was travelling at 15mph when it hit a stationary train at Plymouth Station on 3 April 2016.
The driver “lacked the experience to quickly recognise the identity of the platform occupied”, the Rail Accident Investigation Branch (RAIB) said.
Both trains were operated by Great Western Railway (GWR).
Four crew and 42 passengers were injured in the crash, along with 2 staff on the stationary train
The driver, based in Penzance, had been qualified for less than a year and believed he was going into a vacant platform.
He applied the emergency brake four seconds before the crash with the train moving at about 15mph.
The RAIB also recommended a review of permissive working arrangements, when more than one train is allowed on the same stretch of track.
Simon French, chief inspector of rail accidents, said: “This accident reinforces the need for drivers to take great care when signalled into an occupied platform – assumptions should never be made about the length of platform that is unoccupied.
A GWR spokesman said: “The safety of our passengers and our colleagues is our highest priority, and we will be studying the RAIB’s report closely to make sure that any further recommendations are implemented.”