An update this morning (
from TV5 Monde Info) says:
One tram was stopped at an underground platform serving the railway station.
Another had left was stopped for traffic
For an unknown reason it set off backwards down a slope and was going quite fast when it hit the one behind.
A total of 68 people are recorded as injured, the most serious being "a couple of broken collar bones".
About 100 other passengers were involved and not injured.
Other news sources, including English language ones, should update later this morning.