A shop owner measures a suitcase kept on a rack to be 50 cm × 25 cm...

The proper dimensions of the suitcase are measured when it is at rest relative to the observer. The shop owner measures the proper dimensions as $50 \text{ cm} \times 25 \text{ cm} \times 10 \text{ cm}$.

When the traveller takes the suitcase on the train, both the traveller and the suitcase are in the same reference frame. The relative velocity between the traveller and the suitcase is zero.

Since there is no relative motion between them, the traveller does not observe any length contraction and measures the proper dimensions of the suitcase.

Therefore, the dimensions measured by the traveller remain $50 \text{ cm} \times 25 \text{ cm} \times 10 \text{ cm}$.