Britain had a dream of building a railroad from the Mediterranean Sea to South Africa. Which country was the strongest position to block this from happening?

The correct answer is: Portugal.

Explanation:

Portugal had two colonies in Africa: Mozambique and Angola.

Mozambique and Angola are placed on both sides of the continent and Portugal considered that railway as a threat to sovereignty in Africa. The Portuguese colonies were placed on both sides of the continent, so they would represent an obstacle for the British plans.