I agree with Mark.

Check the gauge of the track using an N scale track gauge. Also check the width of the pilot wheels using said gauge.

The amperage rating of the motor most likely is too low for the decoder. Get a different decoder with a higher rating. Also, use a DMM to ensure nothing was wired wrong.

Probably 3 volts.

Hey now Mark! Sheesh!

99% sure this won't happen.

One exists, and it's made by Fox Valley Models.

http://www.tcsdcc.com/Customer_Content/Installation_Pictures/N_Scale/Bachmann/2-8-0/bachman_n_scale_2_8_0.htm

http://www.tcsdcc.com/Customer_Content/Installation_Pictures/N_Scale/Bachmann/4-8-2%20Light%20Mountain/Bachman_4_8_2_Light_Mountain.htm

The new GP-40s run a lot better than the old ones.

All the 4-6-0s run the same speed.

Change CVs 29 and 67-95 on the diesel to slow it down and match the steam locomotive.

Spray wet water on the ballast after you get it where you want. Wet water is a drop or two of dish detergent with water added. It gets rid of the surface tension of the water and ballast so that the Scenic Cement goes in between the ballast particles.

Check the spacing between the wheels using an N scale wheel gauge.

Most likely the wheels are not in quarter. You will have to figure out which one(s) and put them into quarter by adjusting them slightly.

The new style is called knuckle couplers. They are the industry standard now. Either you will have to convert your older trains to the new knuckle couplers or convert your engine to rapido couplers.