Seven 7% percent, sheesh! It's not reasonable to expect a loco to climb that kind of grade. So there may not be a satisfactory solution to your problem. As someone suggested, you could try adding weight inside the smokebox (front end of the boiler.) Then the loco might make the climb, but it will stretch the tires even more than they already are.
Grooving the front wheels isn't a job for the faint of heart. It requires advanced machining and reassembly skills. Also, it may compromise your ground return, leading to stalling and unreliable operation.
@Paul Kallus I wouldn't run a loco designed with grooved wheels, without a tire in the groove. The loco may not sit level, and the un-tired wheel won't make good contact with the track. This creates an uneven load on the rear axle bearings and drive rods. Depending on what your rails and the wheel are made of, the railhead will muff up the edges of the groove, which could make it more prone to shedding tires in the future. The open groove could potentially snag on curves, switch guide rails, etc., causing derailments. I've been after the manufacturers for YEARS to redesign their chassis with a "bottom plate," removable wheels and axles. Then (assuming spare parts are available) you could just swap the grooved wheelset for a smooth one. So far no joy. 
Bottom line 7% is too steep and not advised. If you MUST have a second level on your layout, make it just that -- level -- and ditch the graded track. A thread comes back to life after two years and the song is the same. Gosh I despise rubber tires!!
