I was reading one of the answers to this question and it implied that Talia Winters and Susan Ivanova were involved in a same-sex relationship on Babylon 5. This came as news to me, as I own the entire series and never once believed their relationship to be more than just a friendship, no matter how much the teenage me might have wished otherwise. Is there any evidence that the two were actually involved romantically?
Yes, they were romantically involved, though it was for only a brief time before Lyta revealed Talia as the spy with the implanted personality and the original personality of Talia was destroyed. The episode Divided Loyalties, where this happens is also indicated as the first, and perhaps only, time when they were intimate.
The show notes and other discussions from JMS make this pretty clear. There were indicators in the episode that some people thought perhaps a kiss was edited out (it wasn't) and where Talia wakes up in bed and appears to reach out for someone (Susan) who was no longer there.
It isn't entirely clear, but this quote from the creator implies that while they were heading there, they didn't actually get physically intimate:
It would have been the first time they got physically intimate. He's not saying that it was the first time.
As for a not-yet-intimate romantic relationship, there was even less officially said, but the fan-notes at the top have a good breakdown that also seems to imply "no" (by way of the first three bullets in particular), but that it was probably where they were headed.
(Note that these are not bulleted on that page, it's just easier to read them like that here)
Yes, Ivanova and Talia Winters were always slated to be a sexually active lesbian couple;
JMS told Sci-Fi Universe (March 1996)
Andrea Thompson (who played Winters) stated in "Creating Babylon 5" that:
An SFX Interview in July 1997 showed that they wanted to explore the relationship further
Claudia Christian also made it abundantly clear that her relationship with Talia was based on a mutual sexual attraction:
JMS (on the excellent Midwinter website) made it clear that they were involved with each other both sexually and on an emotional level;