Bus drivers and mechanics with Gatineau's transit agency walked off the job Thursday morning in the first of what they say will be a series of weekly, one-day strikes.

While the walkout may have come as a surprise to some, the dispute between the Société de transport de l'Outaouais and the union representing workers, Local 591 of the Syndicat uni du transport, has been more than two years in the making.

Here's a summary of how we got here. If you're looking for tips on alternatives to transit, read our STO strike essentials.

What's the dispute about?

The union and the transit agency have been without a collective agreement since Dec. 31, 2014. Though there are a number of issues on the table, wages remains the big one.

Prior to their meeting Wednesday, the STO said it would be putting forth a five-year agreement that called for 1.5 per cent increases in 2015, 2016 and 2017, with the 2015 top-up paid out as a lump sum and some mechanics getting a two per cent increase in 2017 followed by two per cent in 2018 and 2.5 per cent in 2019.

While it's not known if that remained the deal on the table at Wednesday's meeting, it falls short of the 2.5 per cent increases the union had been asking for in 2015 and 2016. The union had argued that since other STO employees, such as supervisors, were already given 2.5 per cent increases in those years, their member should, too.

The STO says the difference in money between the two proposals is about $10 million, though how much of that amount is in one-time lump sum payments and how much is in annual costs remains unclear.

STO president Gilles Carpentier

STO president Gilles Carpentier announced Wednesday that the transit agency was bringing a new offer to the bargaining table, but no deal was reached. (Radio-Canada)

What other issues are on the table?

The union says the STO is trying to curtail the number of employees using their vacation time to book summers off, though it's not clear how the union proposes to resolve this.

For its part, the STO originally asked that maintenance workers spread out their holidays across the whole year to maintain the condition of the fleet, but later came back with a suggestion that student workers could handle simpler jobs such as cleaning while qualified maintenance workers performing higher-skilled work would get a pay upgrade.

The union has also taken issue with shortened breaks for drivers switching buses mid-shift. In its Wednesday offer, the STO proposed creating a joint committee to look at issues with bus schedules.

The STO also proposed reducing the number of annual leave days from the current 14 to 11 in return for a four-day wage premium for employees who are absent three days or fewer.

What's the status of negotiations?

The two sides have been in mediation since November. When a mediator proposed arbitration to settle the dispute, the two sides disagreed over what form that arbitration should take, and Wednesday's meeting did not resolve the impasse.

The transit agency tried unsuccessfully to appeal to the Canadian Industrial Relations Board to recognize public transit as an essential service, but was denied.

They also argued to the CIRB Wednesday that the union had failed to give enough notice for its Thursday strike, but the board again rejected that assertion and workers were able to legally walk off the job.

On Thursday the STO sent a formal invitation to the union to go back to the bargaining table and proposed dates. It's not known if that offer was accepted.

Felix Gendron

Union president Félix Gendron said in January that the STO's suspension of local vice-president Guy Gosselin was an attack on the union. (Radio-Canada)

How did the 2 sides get to this point?

The two sides have been engaged in a war of words for months.

The STO angered the union when it suspended some members who also happened to be union negotiators.

In January employees launched a series of job actions, including wearing jeans, refusing to work overtime or special events, and reporting minor defects on buses, taking them off the road for maintenance.

The STO responded by sending drivers home if there was no working bus available for them.

When will drivers strike next?

The union hasn't revealed a date, but has promised the public will receive 24 hours notice before the next week's walkout.

STO strike

Traffic was backed up on bridges leading from Gatineau to Ottawa Thursday morning as commuters were forced to drive rather than take the bus. (Radio-Canada)