The service disruption on the northeast leg of the LRT on Tuesday was caused by someone stealing copper wire, says Calgary Transit.
Supt. Brian Whitelaw says the theft affected the current flow on the signalling system, causing a safety issue.
Trains had to be stopped for most of the day while crews made repairs.
"Under the right set of circumstances you can actually have a train near two employees on the tracks and that will actually ground through the nearest source, which could potentially be an employee or it could be anybody standing on the rail at the time, or a first responder," said Whitelaw.
The disruption impacted the last three train stations of the Blue Line, or Route 202, and shuttle buses were used to ferry passengers.
Copper theft is an ongoing problem, said Whitelaw, and the number of cases depends on the price it can fetch.
As a result, Calgary Transit is working to make it harder to steal significant amounts.
"We now use very short links so that there's a lot of effort required … which of course increases the chance of being detected" he said.
"At the same time we're looking at some of the newer technologies that actually encase critical infrastructure in concrete."
Calgary's public transit system, including buses and shuttles, saw more than 100 million trips in 2017.
Anyone with information on the theft is asked to call police at 403-266-1234 or Crime Stoppers at 1-800-222-8477.