This Austrains 36 class I've been working on, also suffers from a VERY light front bogie. Rather than adding weight to it, I just removed enough of the spring so that it still applies a very small amount of downward pressure on the bogie. It works well.....spring does not detract from tractive effort but does keep the bogie on the track.
I have 4 of the 36cl, one from the original run & 3 from the current run, on the old layout prior to moving here, which had 1:40 grade with 22" radius curve the original would pull around 6 TOR RU's & van without slipping to a stand with spring in place, removal of the spring gave me 1 extra RU plus an S truck, not significant but still more.
With the full spring in place its possible to see the front driving wheel just off the rail on a straight section of track, no matter level or on a grade, cut the spring in half, & the front driver sits on the rail, seemingly problem solved, however put the model on a curve & you can see the outside front driving wheel is still off the rail, & cutting the spring in half again, barely has the wheel touching the rail but as soon as it moves the wheel comes up & again the loco stops moving.
Take the bogie off & the model lifts the load on the grade & curve, put the bogie back on with the lead weight in place & it moves off no problem, & I do not have the problem with the light bogie derailing anywhere.. From my perspective, far easier to simply remove the spring in the first place & add the weight to the top of the bogie, rather than the muck around of getting just the right amount of spring pressure on the bogie.
I set about adding the weight to the top of the bogie barely 4mm wide on both sides of the cutout, I added thin strips of lead to the inside of the Main Reservoir tanks both sides, a flat piece inside the smokebox, a curved piece to the inside cab roof, & small amount on the floor in front of both cab seats. That added an extra 2 BWH, & another RU to the load on that grade.
While the lead on the body certainly made the biggest difference, the removal of the spring also made some difference. I have also adopted the spring removal from all my other plastic models & it has helped also. The Eureka garratt was typical of light type garratts as it would slip to a stand on the front tank unit, remove the spring & it helped with a couple of extra 4 wheelers, then add the flashing under the keeper plate, with some small shims inside the tank brings the pulling power up to what I see as a prototypical load of BWH on a 1:40 grade for a light type garratt, something mine did not do prior to the mods.
My current layout has a similar grade to that of the old layout, that is 1:40 but this time its on a minimum radius of 28" rather than the 22" of the old layout but its still a tight grade & tests the models.