Rail passengers are being urged to "make their voice heard" by applying for delay compensation every time they are entitled to it.
An increase in the amount of claims would "send a message" to train operators that reliability must improve, according to watchdog Transport Focus.
A report published by the independent body said that as much as £100 million went unclaimed in 2017/18, as just 35% of passengers submitted claims.
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This is despite punctuality on Britain's rail network falling to a 13-year low in 2018, with one in seven trains delayed by at least five minutes.
Some people do not claim compensation because they are not aware they are entitled to it, while others believe it is not worth the effort.
Transport Focus chief executive Anthony Smith said too many passengers are missing out, as he launched the Make Delay Pay campaign.
He went on: "When things go wrong, train operators must ensure every eligible passenger knows about delay repay and how to claim.
"They must also do more to make it easy to claim and automate this process wherever possible.
"To make their voice heard, passengers must claim every time."
The amount of compensation which passengers can receive is based on the type and cost of the ticket held, and the length of the delay.
Most firms offer the delay repay scheme, with some paying out once a journey is delayed by at least 15 minutes.
Transport Focus noted that claim rates are lowest for shorter delays.
Just 18% of passengers claim compensation for delays of 15 minutes or more, rising to 39% for disruption lasting at least half an hour.
The watchdog is calling on train companies to make the process quicker and easier for passengers.
This includes providing choices about how claims can be made and payments sent, and doing more to promote how and when passengers can claim, such as by making announcements on trains.
It also urged firms to establish more automated compensation schemes.
Robert Nisbet, director of nations and regions at industry body the Rail Delivery Group, said: "We want passengers to get the compensation they're entitled to and train companies have helped to increase compensation payments by 80% over the last two years.
"Working together, we're sending personal alerts through Facebook, making more station announcements, and more train operators are offering 'one click' or automatic compensation."