The rail operator Northern has asked the employment arbitration service to intervene in its two-year dispute with the rail union RMT over the role of guards on trains.

On Saturday union members will hold their 41st day of strike action since March 2017, leaving customers with few or no rail services for the 18th consecutive Saturday.

On Wednesday Northern – which managed to run just 40% of trains on time last month – said it had formally written to the Advisory, Conciliation and Arbitration Service (Acas) asking it to use its powers to establish an independent inquiry into the issues at the heart of the dispute over the role of the second person on trains.

RMT immediately dismissed it as a “stunt” and refused to call off the strike. The union’s general secretary, Mick Cash, said: “The call for an inquiry is nothing more than a PR stunt and we have not seen any proposals from Northern rail whatsoever that would make serious progress in terms of resolving the dispute.

“The company are seeking to kick the issue of rail safety into the long grass indefinitely rather than facing up to the fundamental issue of the guard guarantee on their trains and there is no way that RMT will allow them to get away with this chicanery.”

Under its 10-year franchise agreement with the government, Northern is supposed to move many services to driver-controlled operation, where the driver opens and closes the train doors. The agreement says there should still be a second person on the train “for the purposes of customer service and/or revenue control”.

RMT argues that the role of the guard is “safety critical” and that Northern’s plans are all about saving and making money rather than keeping passengers safe. “We don’t need an inquiry; we need genuine and meaningful Acas talks focused on retaining a guard on Northern trains with the full suite of safety and operational competencies including at the crucial platform-train interface,” said Cash.

Richard Allan, Northern’s deputy managing director, said: “The RMT dispute means customers, businesses and the wider economy in the north have suffered the cost of 40 days of RMT strikes, including every Saturday in September, October, November and now December. Customers, employers and colleagues are looking for both parties to resolve the dispute.

“More than 50% of all rail journeys in the UK are made on driver-controlled trains and recently the Department for Transport and Transport for the North publicly confirmed that a second person – in addition to the driver – would be retained on Northern services.

“This second person will provide customer service, including meeting customer needs on accessibility, safety, security, ticketing and information. Despite this, the RMT continues with its strike action.”

Northern said it had promised conductors that their future role would be guaranteed to be onboard trains until at least 2025, and that their current pay would be protected and reviewed annually if RMT called an end to the dispute.

The owner of the Northern franchise, Arriva Rail North, a subsidiary of the German state railway Deutsche Bahn, has applied for compensation from the government for fares lost during the strike. No money has yet been paid out and a decision has not been taken as to the amount, according to a source at the Department for Transport (DfT). They dismissed as “total speculation” claims from RMT that taxpayers would be bailing Northern out to the tune of £24m.

A DfT spokesman said: “We want this strike to end. Guarantees have been offered over jobs and pay and we have confirmed we want to see a second person on Northern trains. With these guarantees in place, no wonder the public are asking why the RMT won’t be reasonable, come to the table and put passengers first.”