An auxiliary subcooling circuit for a transport refrigeration system (TRS) is provided. The auxiliary subcooling circuit may be configured to be driven by a compressor that is separate from a main compressor of a main refrigeration system of the TRS. The auxiliary subcooling circuit can be configured to subcool refrigerant in the main refrigeration system. The auxiliary subcooling circuit and the main refrigeration system may be configured to be driven by a prime mover of the TRS. An engaging device is configured to allow the auxiliary subcooling circuit to engage or disengage the prime mover. Methods to control the TRS are also provided.