I've replaced them and or have made live dummy legacy loco's into Powered ones. some of these LED's have various colors of lights... even when they're the same part #. So I've had to replace both. Replacing the entire Truck is not needed.
but be ready to order at least 2. Make sure there is ample room to install the new LED's in the unit. I've damaged them by removing them from one unit to another. Usually an exacto knife will work or sharp pocket knife. Just to make sure there's enough room to slide the LED's in and out w/out damaging them.
LED's are also available aftermarket but you need to get the proper DC voltage on those. Here's one place to get them: http://www.modeltrainsoftware.com/ledlights1.html
Like John said... make sure your connections are good first before trying to replace them... they're likely disconected (bad wire) or something. LED's should last 9 hrs / day for 40 years??? something like that.
I hope this helps you or just have a local service tech repair it for ya if you're a little uncomfortable with it.