Very good then. The short answer (without knowing your old thermostat model and what wires went to what terminals) is:
B will go to C on the Nest, this is 24 volt ac common and Trane designates this B while all other mfrs. designate it C. Since you had one on the Trane thermostat, you should connect it to the Nest even though it may not be necessary.
X2 would have been for emergency heat on the Trane thermostat and if you had a connection at W on the old Trane thermostat and transferred it to W/Aux/E on the Nest, then you will tape up the end of the X2 wire and tuck it back in the wall. Newer electronic thermostats don't need a separate terminal for W (or Aux) and E where older ones did.
With all that said, the Trane XL16I is a two stage heat pump so you should have connections on: R, Y, Y2, G, W (Aux/E), C and O and that should complete the needed wires for your system with the Nest thermostat. The Nest should configure your system automatically as a two stage heat pump with auxiliary heating.
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