Standard Inside corner
[STCRNR]

Cheap, fast and good, pick two, so the saying goes... or you can pick Klawndyke and get a quality kit that is inexpensive, goes together quickly and easily, ships fast, from a great company with customer service that actually provides service to their customers, and best of all made right here in the USA (you can't get much more American than Iowa, I know because I was born and raised there).
I assembled two of these inside corner kits in a little less than an hour, not counting the time it takes the glue to dry. These inside corners are symmetrical, which I personally love even though they aren't standard for most T-TRAK inside corners, which gives you more real estate to play with.
You do have to mark your own track lines (the kits are newly developed, give them time), drill your own holes for the track power feeds if you are putting them in, and skyboards are a do-it-yourself project, but those are the only "complaints" I have about them. Another great kit from Klawndyke, thank you!