effort ( starting). Braking effort is rated at . Early models featured a bad cab design which was noticed first on CSX as crews reported annoying disturbances such as prime mover noises, traction motors and more. The cab was also known for rattling, leading to the nickname "Thundercabs". As a result, these units are also not approved for leading trains. In 2012, EMD also built four models known as the SD70ACe-P6. These units, unlike previous SD70ACe's, have one inverter per axle on the trucks, rather than EMD's traditional one inverter per truck design. Four of those ( EMDX 1206,1208,1209 and 1210) were sold to Canadian National Railway and renumbered to CN 8100-8103. In 2014, BNSF Railway took delivery of 20 SD70ACe-P4 units, numbered 8500 - 8519. This model was designed with a B1-1B wheel arrangement to compete with GE's ES44C4 model, which has an A1A-A1A wheel arrangement. Both wheel arrangements mean that there are only two traction motors per truck instead of three, those being the ones next to the fuel tank. Two SD70ACe-P4 demonstrators began a 5-year lease at Tacoma Rail in late 2014. On January 1, 2015, the United States Environmental Protection Agency's Tier 4 locomotive emission regulations went into effect. EMD could not successfully modify the SD70ACe's 2-stroke 710 series prime mover to be Tier 4-compliant; thus, the Tier 3 SD70ACe was succeeded by the SD70ACe-T4 in late 2015. However, US production of the Tier 3-compliant SD70ACe continues with Tier 4 'credit units' (new Tier 3-compliant units EMD is entitled to build based on previously-earned emissions credits). Union Pacific and Norfolk Southern are currently the only US roads to own Tier 4 credit unit SD70ACe's. Additionally, EMD has continued building Tier 3 SD70ACe's for Ferromex (4100-4118), Ferrosur (4119-4133), and Kansas City Southern de Mexico (4200-4224) at Bombardier Ciudad Sahagun. These locomotives are restricted to Mexico-only operation and cannot cross the US border (just as Canadian National's newest Tier 3 GE ES44AC's are restricted to Canadian use only). In March 2016, EMD replaced the standard cast HTCR-4 trucks on NS SD70ACe 1000 with the new fabricated HTCR-6 trucks for testing. UP and BNSF plan to test the new HTCR-6 trucks on some of their SD70ACe units also. Union Pacific received 281 additional SD70ACe units, numbered 8824 - 9104, in 2014, 2016 and 2018. These are referred to as SD70AH (T4C), H for "heavy", because they are ballasted to