A transformer assembly, comprising: an electric transformer, particularly a rail vehicle traction transformer; a temperature sensor system; and a computation unit for determining a thermal state of the electric transformer, the electric transformer including a cooling device having: at least one liquid coolant channel arranged such as to absorb exhaust heat from the electric transformer by means of a liquid coolant that is passed through the liquid coolant channel; a heat spreader in thermal communication with the liquid coolant channel for transferring heat from the liquid coolant to a heat dissipation surface of the heat spreader; an air blower arranged and configured to effect an airstream along the heat dissipation surface of the heat spreader; the temperature sensor system including: an entering coolant sensor arranged for measuring an entering coolant temperature of the liquid coolant upstream of the cooling device and for providing an entering coolant temperature signal; a leaving coolant sensor arranged for measuring a leaving coolant temperature of the liquid coolant downstream of the cooling device and for providing a leaving coolant temperature signal; an entering airstream sensor arranged for measuring an entering airstream temperature of the airstream upstream of the heat spreader and providing an entering airstream temperature signal; and a leaving airstream sensor arranged for measuring a leaving airstream temperature of the airstream downstream of the heat spreader and providing a leaving airstream temperature signal, and the computation unit being configured to receive the four temperature signals (S1, S2, S3, S4) from the temperature sensor system, and configured to determine a thermal state of the transformer, in particular of the cooling device or parts thereof, by relating said received temperature signals (S1, S2, S3, S4) to each other. an electric transformer, particularly a rail vehicle traction transformer; a temperature sensor system; and a computation unit for determining a thermal state of the electric transformer, the electric transformer including a cooling device having: at least one liquid coolant channel arranged such as to absorb exhaust heat from the electric transformer by means of a liquid coolant that is passed through the liquid coolant channel; a heat spreader in thermal communication with the liquid coolant channel for transferring heat from the liquid coolant to a heat dissipation surface of the heat spreader; an air blower arranged and configured to effect an airstream along the heat dissipation surface of the heat spreader; at least one liquid coolant channel arranged such as to absorb exhaust heat from the electric transformer by means of a liquid coolant that is passed through the liquid coolant channel; a heat spreader in thermal communication with the liquid coolant channel for transferring heat from the liquid coolant to a heat dissipation surface of the heat spreader; an air blower arranged and configured to effect an airstream along the heat dissipation surface of the heat spreader; the temperature sensor system including: an entering coolant sensor arranged for measuring an entering coolant temperature of the liquid coolant upstream of the cooling device and for providing an entering coolant temperature signal; a leaving coolant sensor arranged for measuring a leaving coolant temperature of the liquid coolant downstream of the cooling device and for providing a leaving coolant temperature signal; an entering airstream sensor arranged for measuring an entering airstream temperature of the airstream upstream of the heat spreader and providing an entering airstream temperature signal; and a leaving airstream sensor arranged for measuring a leaving airstream temperature of the airstream downstream of the heat spreader and providing a leaving airstream temperature signal, and an entering coolant sensor arranged for measuring an entering coolant temperature of the liquid coolant upstream of the cooling device and for providing an entering coolant temperature signal; a leaving coolant sensor arranged for measuring a leaving coolant temperature of the liquid coolant downstream of the cooling device and for providing a leaving coolant temperature signal; an entering airstream sensor arranged for measuring an entering airstream temperature of the airstream upstream of the heat spreader and providing an entering airstream temperature signal; and a leaving airstream sensor arranged for measuring a leaving airstream temperature of the airstream downstream of the heat spreader and providing a leaving airstream temperature signal, and the computation unit being configured to receive the four temperature signals (S1, S2, S3, S4) from the temperature sensor system, and configured to determine a thermal state of the transformer, in particular of the cooling device or parts thereof, by relating said received temperature signals (S1, S2, S3, S4) to each other.