An assembly set for assembling an operational railway traction vehicle, said assembly set comprising: - a non-operational railway traction vehicle including: • a first main traction bogie; • a main traction engine for delivering traction power to the first main traction bogie; • at least one second auxiliary traction bogie; • at least one auxiliary installation space for an auxiliary traction module (18a, 18b) for delivering traction power to the second auxiliary traction bogie; and • a vehicle traction controller for controlling the traction of the operational railway traction vehicle, wherein the auxiliary installation space has a power transmission interface for transmitting power between an auxiliary traction module (18a, 18b) installed in the auxiliary installation space and the second auxiliary traction bogie, and a traction control signal interface for functionally connecting an auxiliary traction module (18a, 18b) installed in the auxiliary installation space to the vehicle traction controller; and - an auxiliary engine traction module comprising: • an auxiliary traction engine; • a traction power connector adapted for establishing a traction power connection between the auxiliary traction engine and said power transmission interface; and • a control signal connector for functionally connecting the auxiliary traction engine to the traction control signal interface; characterised by an auxiliary regenerative brake traction module comprising: • a regenerative brake; • a power connector adapted for establishing a power connection between the regenerative brake and said power transmission interface; and • a control signal connector for functionally connecting the regenerative brake to the traction control signal interface, said regenerative brake being adapted to store brake energy by braking the second auxiliary traction bogie and to deliver traction power to the second auxiliary traction bogie when the auxiliary regenerative brake traction module is installed in said auxiliary installation space. - a non-operational railway traction vehicle including: • a first main traction bogie; • a main traction engine for delivering traction power to the first main traction bogie; • at least one second auxiliary traction bogie; • at least one auxiliary installation space for an auxiliary traction module (18a, 18b) for delivering traction power to the second auxiliary traction bogie; and • a vehicle traction controller for controlling the traction of the operational railway traction vehicle, wherein the auxiliary installation space has a power transmission interface for transmitting power between an auxiliary traction module (18a, 18b) installed in the auxiliary installation space and the second auxiliary traction bogie, and a traction control signal interface for functionally connecting an auxiliary traction module (18a, 18b) installed in the auxiliary installation space to the vehicle traction controller; and • a first main traction bogie; • a main traction engine for delivering traction power to the first main traction bogie; • at least one second auxiliary traction bogie; • at least one auxiliary installation space for an auxiliary traction module (18a, 18b) for delivering traction power to the second auxiliary traction bogie; and • a vehicle traction controller for controlling the traction of the operational railway traction vehicle, - an auxiliary engine traction module comprising: • an auxiliary traction engine; • a traction power connector adapted for establishing a traction power connection between the auxiliary traction engine and said power transmission interface; and • a control signal connector for functionally connecting the auxiliary traction engine to the traction control signal interface; characterised by an auxiliary regenerative brake traction module comprising: • a regenerative brake; • a power connector adapted for establishing a power connection between the regenerative brake and said power transmission interface; and • a control signal connector for functionally connecting the regenerative brake to the traction control signal interface, said regenerative brake being adapted to store brake energy by braking the second auxiliary traction bogie and to deliver traction power to the second auxiliary traction bogie when the auxiliary regenerative brake traction module is installed in said auxiliary installation space. • an auxiliary traction engine; • a traction power connector adapted for establishing a traction power connection between the auxiliary traction engine and said power transmission interface; and • a control signal connector for functionally connecting the auxiliary traction engine to the traction control signal interface; • a regenerative brake; • a power connector adapted for establishing a power connection between the regenerative brake and said power transmission interface; and • a control signal connector for functionally connecting the regenerative brake to the traction control signal interface,