Explanation:
in any scale model (2D as in a map, or 3D as in such a train model) the scale factor is the same for all dimensions.
so, when find the scale factor for one dimension, we can use it to calculate the others.
in our case we get both measurements of the length :
27 meters to 40.5 cm = 0.405 meters.
remember : 1 meter = 100 centimeters
so, to get from large to small, the scaling factor is
0.405/27 = 0.015/1 = 0.015
and so, to get the model width :
3 × 0.015 = 0.045 meters = 4.5 cm (centimeters)
the model is 4.5 centimeters wide.