This new model railbus 39950 has new body and underframe.
The old model 39980 was difficult,like to change out traction tyres!!!
So i wonder if it is same similar problem like the new railbus...!?

Yes...just one.
There is a lot of small details to pick off in the railbus before to change tyre.

Agree. Horrendous...

I have the Railbus 39981 and the manual says:

Changing Traction Tires on the Rail Bus
Traction Tire 7154
• Lay the motor car upside down. Remove body 1 from floor 28
with the help of a flat blade screwdriver. Insert the screwdriver at
the end of the car near the coupler under floor 28 and carefully
„lift“ the floor from the body.
• Loosen screw 11. The upper and lower circuit boards 17 areconnected
by flexible ribbon cables. Be careful when lifting the
upper circuit board 17 from the interior detailing 12.
• Loosen four Phillips head screws 13, gently press the snap in
hooks on the interior 12 inwards with a flat blade screwdriver
and lift the interior detail piece from the floor.
• Do not damage the cable connections from the speaker 14
pressed into the interior detail piece 12.
• Loosen Phillips head screw 18 in order to lift up the lower circuit
board 17. Lift the motor 15 up at the same time as the circuit
board 17. Be careful, do not damage the flexible cable connection
between the upper and lower circuit board 17.
• Lift up the compression spring 23.
• Remove the coupler drawbar 19 and springs 21. Important! The
bronze spring 20 falls easily out of the coupler pocket.
• Turn the floor around and „unsnap“ the pickup shoe 30 according to
the instructions that come with the rail bus. Loosen four Phillips head
screws 29. Slowly lift the floor 28 from the die-cast metal part 22.
• Important! Wheel set 27 without traction tires rests on two compression
springs 26.
• Wheel set 24 can now be removed. Change the traction tires.
• When reinstalling the wheel sets, be careful that the ribs on the
die-cast part 22 grab into the slot in the wheel set mounting shell.
• The rail bus is reassembled in reverse order. Pay attention to the
installation of the current conducting coupling and the correct position
of the interior detail piece, the circuit boards, and the body.
The numbers refer to numbers on the exploded parts diagram.

Video review of 39950.