The usage of Light Rail for the Lisbon Metro Violet Line is NOT an issue!
We have first to understand why it was chosen in the first place. I can’t say for sure, it’s after all a topic of discussion and contention within my circle of friends, but from what I could figure it most likely has to do with cost. As stated, the initial cost rose from 250M€ to 390M€. This is a big jump, but we also have to assume rightfully that the cost of building it like a modern subway would most definitely have risen drastically. The entire circular line project, which contains at most 3km of tunnel and some small viaducts is already set to cost 331M€, and that’s just for 4km of infrastructure! Safe to assume that the sheer cost of tunnelling for 14km would be astronomical.