The Transport Salaried Staffs Association (TSSA) said rail operators had no firm data on passenger numbers during the morning and evening peak periods.
The Association of Train Operating Companies (Atoc) described the claim as “nonsense” and said an increasing amount of information was being collected.
According to the union, unpublished evidence to Parliament’s Transport Committee revealed that rail regulators have to rely on estimates about the severity of overcrowding on UK rail services.
General secretary Manuel Cortes is writing to Transport Secretary Justine Greening urging her to instruct the companies to instal data which would record overcrowding on their trains. He said: “We have a ridiculous situation for passengers in the 21st century.
“They have to travel in cattle truck conditions with no limits on overcrowding while cattle travel under regulations which restrict overcrowding.
“Every Transport Secretary says they want to end overcrowding, particularly in the South East. How can they even begin to do this until they known the exact size of the problem?”
The TSSA will also be objecting to any plans to increase train fares during peak periods.
“British passengers already pay the highest fares in the Western world so the idea they should pay even more to ease overcrowding during peak times is a complete nonsense,” added Mr Cortes.
An Atoc spokesman said: “It’s nonsense for the TSSA to suggest that train companies have no idea how many people are travelling on peak hour services.
“We are collecting more and more information on passenger numbers to identify the busiest commuter lines, using data from ticket sales, carrying out regular passenger counts and using load weighing equipment on trains to calculate how many people are on board.”