Earlier this year it was announced the merger would be delayed amid concerns over the terms and conditions of officers transferring to Police Scotland.
Now the RMT, TSSA and Aslef have joined Labour in calling on transport minister Michael Matheson to re-think the proposals.
The TSSA’s Manuel Cortes said: “Our members have been campaigning against this wholly unnecessary merger from the off. Academics, the police watchdog, police officers, passengers and our members all agree that bringing together the BTP and Police Scotland is unnecessary, unwise and downright dangerous.
“Sadly it’s clear that so far, the SNP is putting nationalist dogma ahead of the safety and well-being of Scotland’s rail passengers.”
Scottish Labour’s Colin Smyth said the merger was “unwanted, unnecessary and uncosted”.
The Scottish Government has said the integration will allow railway officers to access wider support facilities and specialist resources.
A Scottish Government spokesman said: “We are committed to the safe and seamless integration of BTP into Police Scotland following Parliament’s vote to pass the Railway Policing Act. This will deliver continuity of service for rail users and staff and a single command structure, with seamless access to wider support facilities and specialist resources.
“Devolution of BTP was recommended by the Smith Commission, reached through cross-party agreement and integration will also ensure railway policing is fully accountable to the Scottish Parliament.
“We have confirmed that our triple lock guarantee will protect jobs, pay and pensions for BTP officers transferring to Police Scotland. We meet regularly with all partners in the integration and will continue to work closely with them on addressing issues that emerge.”