The Rail, Maritime and Transport union said train guards are being issued with instructions to make security announcements advising passengers to keep bags and belongings with them and report any suspicious activity to a member of staff or British Transport Police.
The union accused rail companies of seeking to remove guards, as it continues with disputes over staffing on Southern, Arriva Trains North and Merseyrail services.
Strikes planned at all three companies were called off this week because of the Manchester bombing.
RMT general secretary Mick Cash said: "Rail staff are required to follow strict procedures to help reduce the risk of terrorist attacks, but if you remove the guarantee of the guard on trains or cut the number of station staff you will reduce the frequency of the security checks and security announcements that could be vital in preventing a terrorist attack.
"We would hope that there would now be an industry and political consensus to say that we should not be taking guards off trains and staff off our stations.
"We are calling for a summit of politicians, the industry, transport police and unions to ensure that security concerns are put at the forefront of decisions on front-line staffing.
"As well as Government ministers, we would hope that summit would also include the new metro mayor of Liverpool, whose transport authority is trying to get rid of guards, and the metro mayor of Manchester, who will no doubt have concerns that Northern Rail are also attempting to introduce driver-only trains."
Paul Plummer, chief executive of the Rail Delivery Group, which represents train companies, said: "Thanks to the hard work of our people and billions of investment, Britain has one of the safest railways in the world and we will never compromise on safety.
"We expect there will be around 100,000 opportunities to work in rail in the next decade as part of the £50 billion-plus investment from Government, Network Rail, train companies and ticket fares that is going towards making journeys better, local economies stronger and giving communities more opportunities as part of our railway upgrade plan."