Len McCluskey, the general secretary of Unite, joined RMT members who are in an ongoing dispute over the role of train guards.
But he and Hilary Benn, Labour MP for Leeds Central, turned out in "solidarity" with the members ahead of a memorial service for Mr Maguire by his grave at Beckett Street Cemetery at 11am.
Addressing the picket line, Mr McCluskey joked: "When there's an MP and a general secretary in a crowd of more than four, it's essential that a speech is made."
He continued: "I'm delighted to be here on this picket line with more regional committee in order to show our solidarity to the fight that you have taken on for safety and to say thank you.
"Thank you on behalf of our members who use the railways and are 100 per cent behind the fight for safety.
"This race-to-the-bottom culture that we have in our nation at the moment has to be stopped and it requires brave men and women like you to stand up and be counted on occasions."
He also accused far-right protesters of "misappropriating" of the Yellow Vest movement, a criticism which comes following rally last weekend.
"It's supposed to be workers coming together against neo-liberalism," he said.
"They will never intimidate a united British working class and that's why we're here today: to say 'We stand in solidarity with you'.
"I'm delighted that my good friend Hilary is with me here. Of course, his dad [Tony Benn] was my personal hero all my life, and I know that the leadership of the Labour Party is fighting very strongly, and fighting shoulder to shoulder, with you."
Mr Benn tweeted: "Standing in solidarity with @RMTunion members at Leeds station fighting to keep the guard on the train. Great to be joined by @LenMcCluskey and the @UniteNEYH regional committee.
Mr Maguire was a key figure in the development of the country’s labour movement and was the informal leader of many worker strikes that shook Leeds in the 1880s and 1890s.
Mr McCluskey said: "I first heard of Tom Maguire, the young, radical trade unionist poet of the late 1800s, when I was a young shop steward on the Liverpool docks.
"I'm really looking forward to giving his memorial lecture in Leeds today."
Leeds East MP Richard Burgon organised the event.
He previously told the YEP: “It’s very important that we commemorate Tom Maguire. He may be relatively unknown now, but when Tom died 125 years ago, aged just 29, the people of Leeds turned out in their thousands to honour him."
Northern has called on RMT to suspend its strike action.
David Brown, Managing Director at Northern, said: “More than 50 per cent 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. Therefore, there is no reason for the RMT to continue its disruptive and economically damaging strikes but despite this, the RMT continues with its strike action.”
He added: “We expect all of our service on Saturdays in January to be extremely busy and are calling on our customers to plan their travel carefully for the coming weekend, check the new timetables well in advance, and make sure they do not rely on the last trains home.”