The British Transport Police Federation said a small number of officers had left to join specialist forces elsewhere in Scotland, while others were seeking transfers to England.
The Federation warned other officers could follow suit if it continues to be kept “in the dark” by the Scottish Government over its plans.
Legislation passed before Holyrood’s summer recess will see BTP’s operations in Scotland taken over by Police Scotland from 2019.
Nigel Goodband, chairman of the BTP Federation, said: “We’ve recently had four transfer and they have gone to non-Home Office police forces such as the Civil Nuclear Constabulary and the Ministry of Defence Police. There are some looking to transfer to BTP south of the Border. They don’t want to be part of Police Scotland and have chosen to go into another force.”
There are around 5,000 BTP officers in the UK, around 200 of whom are based in Scotland. Mr Goodband said it was impossible to know how many officers would leave until there firm details are available from the Scottish Government.
He said: “We’re very much in the dark at the moment as to what’s actually happening and what discussions are being had.
“Right from the outset, we don’t truly feel there has been good engagement. To exclude us because we have different options is an insult and we feel quite aggrieved by it.”
A Scottish Government spokesperson said: “Ministers and officials have met BTP Federation representatives on a number of occasions to discuss our plans and further engagement is planned.
“Maintaining and improving safety and security is our main priority and Police Scotland will maintain a dedicated railway policing unit protected within the broader structure of the force.”
He added: “Integration will provide a single command structure for policing in Scotland with seamless access to wider support facilities and specialist resources, providing an enhanced service to the rail industry and travelling public.”