Sparks were flying on Toronto's Bloor Viaduct Sunday morning, forcing a one-hour closure of trains on a section of the east-west line of the city's transit system as 75 passengers were ordered to evacuate on foot.

Arcing electricity on the tracks between Danforth and Castle Frank. <a href="https://twitter.com/hashtag/TTC?src=hash">#TTC</a> <a href="http://t.co/gPFz3vjmQg">pic.twitter.com/gPFz3vjmQg</a> —@fp_evan

Toronto transit (TTC) spokeswoman Sarah Currie said "electrical arcing" caused the sparks that could be seen cascading downward from the massive bridge that spans the Don River.

Passengers were stranded on the bridge between Castle Frank Station and Broadview Station, when the incident occurred. According to TTC, 75 passengers were escorted by foot to Broadview Station.

Police responding to sparks under the Bloor Viaduct. Some sort of electrical short. If driving be careful. TTC advised Tor. Fire O/S.^GL —@TPSOperations

Firefighters arrived at the viaduct — about 30 metres from the Broadview station — just after midnight and found two large cables burned through to the deck that the train's rails sit upon.

TTC officials say it's difficult to tell what caused the arcing because it is still under investigation, and they will not know if there will be an ongoing issue with the bridge. However, they did say no injuries were reported and the bridge re-opened about an hour later.

Bloor Viaduct just started exploding with giant sparks of light, fire trucks and police cars <a href="https://twitter.com/hashtag/Toronto?src=hash">#Toronto</a> <a href="https://twitter.com/metromorning">@metromorning</a> <a href="https://twitter.com/CTVNews">@CTVNews</a> <a href="https://twitter.com/CBCNews">@CBCNews</a> —@alice_lenore

Definitely terrifying scene on the Bloor bridge (viaduct) between Castle Frank & Broadview station. <a href="https://twitter.com/TTCnotices">@TTCnotices</a> <a href="https://twitter.com/hashtag/TTC?src=hash">#TTC</a> <a href="http://t.co/hbD79cvglj">pic.twitter.com/hbD79cvglj</a> —@KrisReports