The winter of 1988 began with intensive presentations of plans to build a metro. Communist Party messengers encouraged the masses of Latvian workers, claiming that Riga would have the most modern form of public transport—a metro—just like all significant cities in the world. It would be built with “Moscow’s money”, attracting thousands of imported workers. Thus, an “invasion” of migrants would be massive.

This aspect of migration raised the first doubts and, for the time being, only internal protests. However, there were other aspects: Riga’s complex subterranean environment, the Old Town, which could collapse during construction, and much more.

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In short, Riga is too small to have a metro built in it: it would be inefficient for the city. Other downsides included the city becoming a perpetual building site, the risks of subsidence, and a dramatic influx of crowds of migrant workers. In the Soviet Union, there was an unwritten rule: every city with a population exceeding one million was entitled to a metro. At the time, Riga was short by 150,000. But Moscow had a special attitude towards Riga: Latvia had always been one of Moscow’s favourite republics... Unfortunately, we still feel this “love” even now. If construction of the metro had gone ahead, it would have become the most expensive in the USSR: the cost of a single kilometre would have reached 26 million roubles.
https://neatkariga.nra.lv/komentari/elita-veidemane/412871-rigas-metro-aizgaja-pa-gaisu-pirms-35-gadiem