How do you sync the LEGO high speed train remote?
I'm going to take a stab at this. Presumably you're asking how to make the train remote properly control the train. There are two things to consider: 1. Which channel to use, and 2. Motor direction.


Channel: The train remote features two wheels which can be used to control two motors independently. One has a blue line and one has an red line. These correspond to the blue and red markers on the IR sensor. Also, the IR sensor and the remote have an red  sliding tab with numbers 1-4. These need to be set to the same number. So check on your IR sensor and see which number your slider is set to, and which side (blue or red) your motor is connected to. Then make sure you set the slider on the remote to match the IR, and use the appropriate wheel.
Direction: There are two ways to control the direction the train goes when you turn the wheel. One way is to flip the polarity switch on the train battery box. The other way is to flip the direction switch on the remote. If you find the train goes backwards when you turn the wheel meaning it to go forwards, just flip this switch.


Aside from these cases, there is nothing else to configure about the remote. As long as your batteries are fresh and your train is wired up properly, it should work just fine with any combination of sensors and remotes. You can even use one remote to control multiple trains as long as each one is on a separate channel/colour.