It sounds like you’re running into the problem of the timeline processing engine disagreeing with you.
When you tap that segment and “change” its type to its same type, to split it out as a separate timeline item (aside: I really need to add that “Promote to timeline” button), that will be succeeding, but then the timeline processing engine will be disagreeing and merging it back in.
The reason why it’ll be doing that (assuming I’m correct in my guess of what’s going on) is that the tram segment won’t be meeting the conditions for what qualifies as a “keeper” timeline item. In order to stop the main timeline view becoming a cluttered, unprocessed mess of lots of brief segments, each timeline item needs to meet “keeper” conditions.
For Visit items that’s I think 2 minutes minimum duration, or having a manually assigned/confirmed Place, in which case it’s allowed to be shorter than 2 minutes.
For Path items, eg this Walking item, and the Tram segment within it that you’re trying to split out, the conditions are … I’ll have to go check … ok, it’s minimum 60 seconds duration, and minimum 20 metres distance. So if the Tram segment is less than 60 seconds, or less than 20 metres, it won’t qualify, and the timeline processing engine will overrule you and merge it back in.
Although in your screenshot it looks like surely it must meet both those conditions. Unless perhaps the way that the samples were recorded resulted in it falling just below 60 seconds, which seems the most likely case. Certainly it looks more than 20 metres.
Unfortunately there’s no way around this one. For Visits you can get around the “keeper” requirements by manually assigning a Place (ie confirming the place assignment in the Edit view). But for Path items the processing engine is just going to win. I haven’t come up with a good solution for this yet.
You can however use a loophole! If you change a short bit of either of the walking segments on either side to be stationary, eg a little bit of stationary to indicate getting on and/or off the tram, you can then extract that little bit of stationary out as something like “Tram Station”. Then that brief little Tram Station visit will meet the “keeper” requirements, due to having a manual/confirmed Place assignment, and it will act as a buffer between the walking item and the tram item. The processing engine can’t overrule you in that case.
A bit of a tedious hack, I’m sure you’ll agree. But until I figure out some way to tell the processing engine “I want this short bit of travel kept as a separate timeline item, so don’t overrule me” it’s the best we’ve got.
That little loophole/workaround also often ends up creating better timeline data, because typically we do want to have a little visit there to mark the stations at either end of train/tram/etc trips. I’ll often manually extract brief stationary segments like that just to preserve that context. Like, if I get off a train quickly, and then end up walking away from the station in the same direction as the train line, there won’t be enough “stationary” or clustered samples for the processing engine to “see” a visit - I really did barely stop moving, and go straight from train to walking. But by converting just a couple of samples of the start of the walk to “stationary” I can fake up a brief little visit, to add that contextual information to the timeline.