It sounds like you have a short circuit somewhere. Remove all trains from the track and try the power again. If the short still exists, physically divide your layout into 2 sections. Apply power to each section, and see which section the short occurs in. When you find it, divide that section of track in two, and check both sections for the short. Continue dividing the track into smaller sections until you find the short.

Look for Christmas tree tinsel across the rails, or a bad fishpaper insulator where the rails are crimped to the ties.

Larry