Wheelslip problems.

Hello everyone, I have built a LEGO MOC of a NS 3700 steam locomotive, but it has some wheelslip problems. The locomotive starts to have trouble when pulling 1+ kilos. The locomotive plus tender weigh only around 750 grams, with batteries in the tender it is a little bit heavier. However I have no space in the locomotive to add weight for traction, so I was wondering what the best choice is, remove 1 motor for space for weight, or find a replacement for the official LEGO rubber traction...