Complex sentence: A major upgrade, which some sources identify as the beginning of the modern tram network in Minsk, started in 20 January 1928 and concluded on 13 October 1929, which marked the formal opening of the new system, now using electrically powered trams.
Split into simpler sentences: A major upgrade, which some sources identify as the beginning of the modern tram network in Minsk, started in 20 January 1928 and concluded on 13 October 1929. This marked the formal opening of a new system, now using electrically powered trams.