DAR ES SALAAM, June 4 -- AS the sleek electric train glides quietly out of the station, its polished silver coaches and orange stripes reflecting the morning sun, Tanzanias Standard Gauge Railway (SGR) offers more than just a journey, it tells the story of a nation on the move.

From the bustling platforms of Dar es Salaam to the wide-open plains stretching inland, the modern train cuts across landscapes once defined by slow, winding travel, ushering passengers into a new era of speed, comfort and connectivity.

Yet, as the train hums forward along the steel tracks, another set of figures has been rising behind the scenes. The massive infrastructure investment powering this transformation has come with a price: A steadily growing national...