Traction of the ICE 1 trainsets is achieved by the eight driven axles of both power cars. Because relatively few axles have traction, the steepness of the grade the trains can safely negotiate are limited. The ICE 1 is limited to grades of about 3.5%. Steeper grades cannot be negotiated safely, because the wheels could slip due to the low adhesion under adverse conditions, which may lead to a stopped train becoming stuck on a grade. This is one of the reasons the ICE 1 is not homologated for the Cologne-Frankfurt high-speed rail line, which has grades of up to 4%.