Rail freight drivers are threatening to boycott hundreds of trains in a row over the state of cabs, says the drivers' union Aslef.
The Class 66 loco is a commonly used freight engine
The union said its members will refuse to drive the Class 66 locos from 18 June unless conditions are improved.
Drivers are complaining the cabs lack air conditioning and proper seating and are noisy and unhealthy.
One rail freight firm, EWS, said it was "working closely and positively with Aslef" on the issue.
There are hundreds of Class 66 locos in operation, which make up a large proportion of the UK's rail freight fleet.
Keith Norman, Aslef's general secretary described the cabs as "unhealthy, unsafe and unsatisfactory".
Freight firms have been told that Aslef members will refuse to work on the locomotives on health and safety grounds from the June deadline.
The union said no positive progress had been made despite trials for new seats and air conditioning.
Aslef also warned safety was being compromised because drivers' concentration was affected by heat and other poor working conditions.
Millions of tonnes of goods are carried by rail each year, saving on lorry journeys.
BBC labour affairs correspondent Stephen Cape said research had shown that in July last year when the weather was hot, the number of times a driver jumped a red signal increased.
The union said this showed safety was being compromised, our correspondent added.
A spokesman for EWS said: "The Class 66 locomotive has been used in Britain since 1998, and is used by the four main rail freight operators in Britain.
"EWS has outlined to Aslef proposals to improve the cab environment of these locomotives, and is awaiting feedback from them on these proposed improvements for EWS drivers."