My recently acquired Flying Scotsman Marklin 39969 struggles to achieve traction even on the slightest of grades (under 2%) just on its own. I can see the wheels slipping so got replacement traction tyres 7152. Surprised to find they are rather small. Double checked the manual and part number is correct. Was quite a mission to replace with the new tyres. However hasnt improved and possibly i feel its even worse with the new traction tyres. This is the only Loco that has any issue on the track. None of my other steamers or any others have this issue. Help?

Does it struggle on the whole incline or only at the change of incline?
Many modern steamers have issues at the beginning of an incline when to slope changes.

Yep, I read that.
At club meetings, I saw a streamlined BR 05 struggle where many other locos had no problems.
With BR 05, the traction tyres were hanging in the air and no longer touched the rails.

Many thanks for the very useful responses. Its much appreciated.
So to answer,
The new tyres are way smaller than the old. Had to really struggle to get them in.
Have removed the new and put back the old after cleaning them a bit.
The slip begins at the start of the incline which incidentally is on a curve.
With 3 freight (empty) cars and a track cleaner, with the old tyres back, it slips at 3 specific points on the track. 2 on curved incline and one on a straight level. It will slip for a second or 2 and then continue. This is at very slow speed. At close to full speed it distinctly is jerky at these specific spots. So I'm guessing thats the slip. The wheels are not getting raised and sitting on the track all the time as much as I can tell. Took a phone video up close and doesnt seem to show any wheel up. However it cant be ruled out as the video is a bit shaky, unsteady hands!
The AC/DC Black Ice loco which is similar runs with no issues at all. Also an older BR 86 and BR 50.
Could the tracks at these specific places be a bit more smoother? Will see if I can swap out these tracks.

Why is the new traction tyre a different size to the original? Have posed this question to marklin.
With the new traction tyre stretched so tight its almost like no traction.
The AC/DC loco spare list on the marklin site (item 7) doesnt list a part number (just found out)!

Many thanks for that. To clean the tyres I used a q tip with a bit of rubbing alcohol. Running the steamer alone has the same effect, which is what puzzled me. The tyre size (7152) I believe is the correct size in the pack. Just that its rather small. Comparing with the Marklin tyre sizes 7154/53/52 they seem to be correct. 54 smallest and 52 being the larger. See attached image. I just looked up your 39241 which seems to be similar and the part number for the traction tyre is 7152. I just cant figure out why its so much smaller even being rubber and stretchable. This is what makes it the hardest to replace. EDITED to add, replaced the 2 130 curve sections with a pair of new tracks and well running the loco by itself no more slip here and with 3 freight cars no slip. Need to dig into a few boxes to find more spare track.

The track cleaning car (marklin) has felt pads and these are soaked on rubbing alcohol, which i regularly run whenever a train is running. Usually at the end of the consist. Will give the removed tracks a good clean as well. But I suspect it’s just unique to this loco only. I also use the Noch felt pads attached to the bogies.

Just as a check I would be tempted to clean the track with something clean and with alcohol - the felt pad if removable or one of the bogie pads or something else(all of those create drag) and see if they pick up anything from the tracks. Hopefully nothing but I am always amazed when I clean track at how much dirt or corrosion is removed.