The scene Sunday morning  outside of Union Station as riders are taken around a work zone area on shuttle buses. (Luz Lazo – Washington Post)

A contractor who was critically injured early Sunday in a Metro tunnel explosion has died.

Two other workers were critically injured in the explosion and fire that broke out in a Red Line tunnel where track work was underway as part of Metro’s major system rebuilding project.

A Metro spokesman said leaking hydraulic fluid on an underground vehicle apparently was ignited by welding equipment, causing an explosion and fire.

Until the incident, which occurred near Union Station shortly after midnight, it appeared that reconstruction had gone relatively smoothly, despite some riders’ surprise at the closure of five key stations, from Farragut North to Union Station.

Track work was suspended early Sunday after the incident in the tunnel about 400 feet from the Union Station platform in the direction of Judiciary Square.

Passenger service in the area had been suspended long before the incident; no passengers were injured.

Metro crews were working Sunday morning to put rail back in place on the Red Line near where the incident occurred so normal service could resume Monday morning, Metro said.

Five key stations — Farragut North, Metro Center, Gallery Place, Judiciary Square and Union Station — were all closed as of 10 p.m. Friday for major rebuilding as part of a $5 billion capital campaign. Riders were able to use the Blue and Orange lines at Metro Center and the Yellow and Green lines at Gallery Place. Plans had called for the five stations to remain closed until the start of service Monday morning.

The shutdown affected nearly one-quarter of passengers who travel over the weekend. Many riders whose trips took them through the downtown area had to get off the train, take a free Metrobus around the closed area and get back on the subway to continue their trip.

Sunday morning, few riders or Metro employee, who were directing people to the shuttle buses, knew of the accident from the early morning hour.