Just came across this.
My Stainz is also from a starter set, a Marklin, so newish. I added weight to get it up to 1.9kg, which transformed its performance. (I haven`t removed the tyre, simply because it is performing so well, so I thought, leave well alone). Here it is right now, as I speak, hauling three 4 axle coaches (albeit with ball bearing metal wheels, which are expensive but give a beautiful ride). I have two moderate gradients, and one S-Bend - R3 - and the Stainz is happily running around the circuit on analogue throttle notch 2. Which just increase my exasperation with my Rugen (per my other thread) for which I bought these coaches!