Since we do not know which Big Hauler agaglione has, we cannot be absolutely sure what the situation is. In the version 2 Chassis it was normal for the wheels to turn and the motor to turn with them when turned by hand or pushed on the tracks. In versions 3,4,5 and 6, if the wheels turn by hand or the motor runs and the loco won't go it is definitely a gear problem. Versions 2,3,& 4 were known for loose or broken gear boxes. Most likely the problem is a loose or cracked or stripped gear as Kevin stated earlier.
I have am now getting reports that some version 5 chassis have been discovered to have a cracked main drive gear. I just tore one down that was given to me by a friend, and it is clearly the large axle gear that has the crack allowing the gear to turn on the axle. I will part it out since I have no need for another Annie. The unit in question was in service since they first came out in 2000, and it did run light although poorly, and when cars were added the motor could be heard turning while the loco was stopped. So Far my 3 Version 5 chassis have not had problems, one in regular use since 2001. I find this amazing for a model steam locomotive, since everything mechanical will eventually wear out in normal use.
Here is the latest Chassis version:
Big Hauler 4-6-0 and Anniversary 4-6-0 Chassis Version Identification by Bill Canelos
Copyright 2002-2014 by Bill Canelos All rights reserved.
Version 1 1988-1990 Battery Powered R/C Units.
Version 2 1990-1994 Smooth Bottom Cover You can turn drivers by hand and motor will turn.
Version 3 1994-1998 Smooth Bottom Cover You cannot turn the drivers by hand on this and all the following versions.
Version 4 1998-1999 Large wide hump dead center between the rear drivers on bottom cover.
Version 5 2000-Pres 3/8th inch wide hump offset between the rear drivers and a plastic lubrication plug the size of a dime to the rear of the hump.
Version 6 2011-Pres New Chassis for Annie only, has metal gears and new front truck identifiable only by the new front truck
Note that the dates overlap and as of Dec, 2013 have not been fixed more precisely, but all new sets introduced in 1999 with the VHS instructions were version 5
The following sets released in late 1999 were Version 5 Chassis 90031, 90032, 90033, 90034, 90035 and 90037.
All Big Hauler 4-6-0's manufactured after 2000 have the version 5 chassis, all new sets after 2000 have it as well. All Anniversary 4-6-0's also have the version 5 chassis until 2011 when they were upgraded to version 6.
As of Dec 2013 Sets have version 5 chassis but all new Annies produced from late 2011 have the version 6 chassis it remains unclear if version 6 chassis will be included in sets at some future date.
One we are able to be sure of the problem agaglione is having then we can perhaps give some useful advice.
Cheers & Beers!!!

Bill