You might try this:
First, make sure all track surfaces are clean from old moulding.
Then, cut new T moulding longer than needed (It Will Shrink in time), 1/4 -1/2 inch extra per foot.
Then put into a bucket of very, very hot, and quite soapy water for a few minutes… the hotter the better, but not boiling.
Then, working fast before it even begins to cool, slide it into the track and force it in as you go... Remember, it's longer and you have to fit it all in.
As you get to the end continue forcing it until it’s all in place. As you near the end, if it begins to "bubble out" of the track, try forcing even more first. If that doesn't work, cut some of it off.
Using this method, as it shrinks, and it will, it won’t leave any unsightly gaps.