Passenger Focus made the demand as it emerged both the West and East Coast main lines would be crippled over Easter.
Train operators have warned journey times between Glasgow and London will nearly double, from four hours and 32 minutes to eight hours, over the bank holiday weekend.
Northbound passengers will be expected to take a replacement bus from Preston to Glasgow on Easter Saturday and Easter Sunday. Those going south will be bussed as far as Carlisle before catching a train.
Edinburgh to London trains will face 30-minute delays the same weekend, although passengers will stay on trains.
David Sidebottom, of the official watchdog Passenger Focus, said: "Rail passengers would appreciate not having to pay full fares when some of the journey is by replacement bus.
"Engineering work can cause major inconvenience, especially from replacement buses – although passengers do welcome investment in improving rail services."
Train operating companies charge normal fares on days they have planned engineering works.
Operators do offer compensation for delays – but only unexpected ones. They do not automatically provide cheap tickets for anyone buying a journey subject to planned engineering works or replacement buses.
Yet the train operators themselves will claim compensation from track and infrastructure owner Network Rail for delays.
Expert Ian Baldry of IBPTS travel consultants said: "Passengers get irate, confused and disgruntled about engineering works as service information can be very limited.
"Delays to replacement buses worsen delays and friction between passengers and operators. Leisure passengers are being hit at Easter as the industry targets work when fewer commuters travel."
Bruce Williamson, of rail campaign group Railfuture, said: "Train passengers do not like being put on buses – and we don't believe the industry is keeping replacement bus use to an absolute minimum.
"Customers get frustrated by journeys being changed by engineering works."
Network Rail said most routes would work normally over Easter, but 43 separate engineering works have been planned for the weekend across the UK network.
Train companies, however, believe it is better to plan such work in advance and to carry it out on quieter weekends. Dozens of replacement buses, voted passengers' No 1 dislike, are planned.
Some 600,000 Scots are expected to take trains between Good Friday, March 29, and Easter Monday, April 1.
Network Rail said: "This Easter 6300 more trains are scheduled to run UK-wide compared with last year. Massive strides have been made in recent years in reducing the impact of our work on passengers.
"But we still advise anyone travelling over Easter to check to see whether there are any alterations."
The Easter misery will not only affect big cross-border routes.
All ScotRail trains from Glasgow Central to Barrhead and East Kilbride will be axed and replaced by buses from Easter Saturday to Monday.
Passengers who fall victim to unexpected delays can claim compensation.
This varies from a free soft drink – subject to availability – on short delays, to full refunds for substantial disruption. ScotRail, which runs all domestic Scottish services, offers its season-ticket holders discounts on their next tickets if performance is poor during the previous month.