Trane air handler twe036c140a0 create “c” wire
Based on this diagram of the control page 4 https://ww2-secure.justanswer.com/uploads/Wvfan79/2012-07-13_122319_twe036-sf-1d.pdf the "c" source is the blue wire leaving the transformer, which becomes terminal 6 at the low voltage terminal block. 

If that diagram doesn't reflect your model or current control board then you would need to identify the two 24v leads from the transformer, one should attach to a fuse, the other should be the C source.