Wiring Lionel Gate Crossing 6-12062 via accessory power?

External power is best. Since you are new search the forum for “common ground” and “insulated rail”.

This accessory does not lend itself to being powered externally without some modification.  In short you need to open it and disconnect the wire from the center rail.  Connect that wire to your accessory power.  If you are using the same transformer for track and accessories, you only need the hot wire from the accessory post.  If you are going to use a separate transformer make sure they are phased (check Search for explanation, if needed).  As Rod suggested you can use a isolated rail to extend the trigger track, so the gates will close a little before the train gets there.

Welcome to the forum.

Thanks, gentlemen! A straightforward explanation on both parts. I'm assuming this approach negates anything other than the two isolated sections that came with the crossing (or more insulated tracks if desired). Please let me know if Im wrong. 

Not sure how far you want to go.  You can easily extend the insulated section on either (or both) side(s) of the gate.  If you take a standard 10" section and turn it over you will see silver colored strips connecting the two outside rails.  Use a small screwdriver to lift the tabs and remove those pieces and press the tabs back down.  Insert this 10" section next to the gate section and then one of the 5" insulated sections.  Since the outside rails are no longer connected you have extended the trigger for the gates by 10".  This can be done to any piece of FasTrack and will work as long as you have an insulated section at each end.  Alternatively, in addition to removing the connectors from the bottom,  you can remove the rail connector from one side at the end of the track or cut through the rail and on side and make your own insulated track.  It's a good idea to mark any altered track section so you can recognize it later on.

If I recall when you flip the crossing over you will see a red wire soldered to the center rail.  If you cut that wire and run extension wires from each side of the cut wire to a remote toggle switch you could then choose when to have the gates/sound/lights operational and not worry about a separate power supply.  

Again, thanks to all of you for help. I had to return the first crossing as it was defective. Supposedly I'm getting another one (from a different source) that's of new design. Let's see what happens! 

It's very important on this accessory to make sure the insulated rail is on the correct side for this to work.  The two 5" insulated track sections must have their outside rail with the "cut" on the same rail marked as "insulated" when looking at the bottom of the crossing.  Otherwise, the gate crossing will go down whenever there is a train on the layout. 

The new design just simply changed the mechanism of how the gates are lowered/raised.