Final answer:
Taking a stretch when making up trains is to test the forces and pressures created by thermal stress on the railroad tracks.
Step-by-step explanation:
The practice of taking a stretch when making up trains is to test the forces and pressures created by thermal stress on the railroad tracks. Thermal stress occurs when there are extreme temperature changes, causing the tracks to expand or contract. By stretching the tracks during train composition, it helps ensure that they can withstand the forces and pressures that they will experience during operation.