Rail users are paying higher fares than they need to because of poor training for station and National Rail Enquiries staff, says Which? magazine.
Undercover researchers from the consumer watchdog asked staff for advice on fares for 150 train journeys.
Which? claimed 59% of station clerks and 43% of operators on the National Rail Enquiries line failed to advise passengers of the cheapest options.
Train operators said its research was based on "unrealistic scenarios".
In nine out of 10 cases, a "passenger" hoping to make two round trips from Oxford to Cardiff in a week was advised to buy two return tickets costing £200 each, Which? said.
Only one clerk pointed out that buying a weekly season ticket would save the passenger £112.
Which? also criticised the nationalrail.co.uk website for failing to give cheaper fare options.
Which? also surveyed 1,515 people who had travelled by train in the last year and found half were confident they knew how to get the best available fare.
Which? chief executive Peter Vicary-Smith said: "Train operators seem blind to the fact that their ticketing systems are too complicated.
"If people who do this for a living can't find the cheapest fare, what hope do passengers have?"
He said clearer signposting was needed to help passengers find the best fares.
Alexandra Woodsworth, the Campaign for Better Transport's public transport campaigner, said: "Train passengers shouldn't have to work so hard to find a decent-value fare.
"There is little point in having cheap fares on offer if the system is so complicated that even ticket office staff can't help you find them."
But a spokesman for the Association of Train Operating Companies (Atoc) described the Which? report as "seriously misleading and misrepresentative".
He added: "Asking 150 questions on unrealistic and obscure scenarios cannot come close to giving a representative view of the 1.3 billion journeys that are made every year by train.
"The researchers haven't actually asked for the cheapest ticket in all the scenarios and even where they have done, they have explicitly excluded the cheapest fares."
He added: "Even though the magazine only surveyed 1,500 people, it claims its research is more accurate than that of the independent watchdog Passenger Focus, which twice a year samples more than 27,000 passengers and whose latest survey found that 84% were satisfied with train services."
A Department for Transport spokesman said: "Train operators are obliged to sell the most appropriate through fares for their journey but it's clear more needs to be done to ensure their customers are offered the best deals available.
"That's why we have asked Atoc to put forward proposals to make ticket retailing easier for passengers and more efficient for the taxpayer."