If passed by MSPs, the Railway Policing Bill will see British Transport Police (BTP) in Scotland subsumed into Police Scotland.
Labour and the Conservatives have previously opposed the move.
The Scottish Government says the merger would make railway policing more accountable.
The RMT, the union representing transport workers, has warned it will put rail passengers and workers at an increased safety risk.
Meanwhile, rail operators have warned significant work must be done now to ensure a “seamless” transition.
Scottish Labour’s justice spokeswoman Claire Baker said: “The SNP has repeatedly ignored the advice of experts on the merger of British Transport Police and is intent on railroading through an expensive plan to fix a problem that doesn’t exist.
“Experts from across the field, including trade unionists and BTP officers, have warned of the risks this merger places on passenger safety.
“BTP officers are trained in policing our railways and, particularly after the recent terrorist attacks, we shouldn’t risk losing that expertise.
“It seems that no matter how many people advise the SNP government that this is the wrong decision, ministers continue on regardless.”
A spokesman for transport minister Humza Yousaf said: “As Police Scotland has made clear, specialist railway policing expertise and capacity will be maintained and protected within the broader structure of Police Scotland, with improved access to wider support facilities and specialist equipment, providing an enhanced service provision to the rail industry and travelling public.
“The Scottish Government has publicly committed to providing a ‘triple-lock guarantee’ that secures the jobs, pay and pension conditions of railway policing officers and staff in Scotland.”
RMT General Secretary Mick Cash said: “The Scottish Government’s proposals to abolish the British Transport Police put nationalist ideology before the safety of workers and passengers on Scotland’s railways.
“RMT call on MSPs to oppose the Bill but we also back a series of amendments which will provide absolutely essential safeguards for workers and passengers, including an independent review into the effectiveness of the legislation and the full involvement of the trade unions.”