Three rail unions have criticised the Scottish government's plan for Police Scotland to takeover the work of the British Transport Police in Scotland.
Justice Secretary Michael Matheson wants to integrate railway policing when responsibility for this specialist service is devolved.
The RMT, the Transport Salaried Staffs' Association (TSSA) and train drivers union Aslef all oppose the change.
It is understood BTP had hoped to continue providing the service.
The force had hoped it would operate with oversight from Holyrood rather than Westminster after control was devolved to the Scottish Parliament, as recommended by the Smith Commission.
BTP is a cross-border police force which employs more than 200 officers in Scotland.
The Rail, Maritime and Transport (RMT) union's general secretary Mick Cash said: "It is sheer arrogance on the part of the SNP government that they are ignoring the advice of the BTP and those who work in the railway industry and are forging ahead with plans to merge this important and distinctive policing operation into the wider force.
"Skills and expertise in dealing with the specialist policing needs on the railways would be lost for ever and would result in an inferior policing service which would impact on staff and passengers alike.
"RMT stands alongside our sister rail unions in opposing these ill-conceived and dangerous plans."
Aslef's organiser in Scotland, Kevin Lindsay, said: "BTP in Scotland understands the way the railway works and has the special expertise to deal with the special problems encountered on the railway every day.
"We don't want the BTP to be scrapped and its services delivered by Police Scotland".
The TSSA's general secretary, Manuel Cortes, said: "The SNP leadership know nothing about the practicalities of running a safe and secure railway.
"If it ain't broke, don't try to fix it. The present system is far from broke and doesn't need fixing."
The Scottish government has been lobbying the UK department for transport for control of railway policing since at least 2011.
Under one possible model, the BTP in Scotland could become a specialist division of Police Scotland, known as the Scottish Transport Police.
The Scottish government has promised to consult on the new arrangements to find the best way forward.
A Scottish government spokeswoman said: "The Scottish government is keen to ensure that the specialist skills and knowledge of the railways held by BTP officers is maintained.
"Over the course of 2015, we will continue to engage with all stakeholders including the BTP, the BTP Authority, the BTP Federation and the rail industry on how this can be most effectively achieved within our national Police Service."
BTP is funded by contributions from the rail industry. In Scotland that includes millions of pounds in public money through the Scotrail franchise.
A spokesman for the Rail Delivery Group, representing rail operators and Network Rail, said: "We support the desire for the Scottish people to have greater local accountability and want to see the policing of the railway in Scotland maintain its close links with the rest of the industry.
"With over 21 million cross-border rail journeys a year and rising, it is vital that any changes are very carefully thought through to ensure that police forces continue to closely co-ordinate activities across the network and build on the good work already in place."
The three main opposition parties at Holyrood have raised concerns about the Scottish government's approach and called for a wider debate about the future of railway policing.
Labour said ministers were not honouring the agreement reached by the five parties represented on the Smith commission on further devolution.
Labour's justice spokesman, Hugh Henry said: "We agreed that the functions of the British Transport Police should be devolved with accountability to the Scottish government and Scottish Parliament.
"We did not agree to the BTP being scrapped and services being delivered by Police Scotland.
"Dissolving the BTP into Police Scotland is yet another attempt to centralise services and reduce accountability".
Powers over railway policing are expected to be devolved to Holyrood by the next UK government after the general election in May.
Police Scotland could takeover the work of the BTP by the end of 2016.