A permanent stumbling block in Kdenlive has been that in the past tracks have been opaque. If you put, say, a title clip with opacity on one track, and your video clip on the next track below ... then you only get your title clip, yet no transparency. New users tore their hair because they expected tracks to work like layers in graphics applications, such as The Gimp, and others.
Transparent/Opaque Track UI
|The fourth track icon: track transparency.|
With recent Kdenlive versions, Kdenlive now defaults tracks in new projects or when adding a new track to be transparent. The bottom-most track always is non-transparent, for obvious reasons.
A transparent track is simply signaled by a fourth track state icon, in form of a checkered square. This is the new default behavior.
If this square instead is uniformly gray, then the track reverts back to the old behavior and its opaque (non-transparent) then.
You can still use transitions as you like, event on transparent tracks. The track transparency only kicks in those regions where you do not specify any explicit transition.
Behind the scene, Kdenlive adds an invisible cairoblend transition to each track that is set to transparent. That is, invisible in the timeline, yet effective when you check the final render outcome in the project monitor.
Beware, however, that Kdenlive up to 15.12.2 has a bug which prevents the track transparency from working correctly in all cases. This has been only fixed as of 2016-02-26, see bug #359150. Before, transparent tracks worked only for two clips atop each other, but not for three or more clips&tracks.
Unfortunately, there's another catch, at least for the time being:
Beware that many distributions still ship the latest official frei0r plugin release package that unfortunately has a bug when handling transparency in the cairoblend transition. The cairoblend transition is the transition used by Kdenlive for transparent tracks. Bummer.
As a work-around, don't rely transparent tracks yet, but instead use explicit affine transitions. Also avoid cairoblend transitions until your distributions ships fixed frei0r plugins.