The UK Defence Ministry's report states this.
On March 18, Russian President Vladimir Putin announced the construction of a railway line from Rostov-on-Don in southern Russia through the temporarily occupied territories of Ukraine to Crimea. According to Putin, the new line should eventually reach Sevastopol and is intended to duplicate the Kerch bridge.
Putin also said that the first completed section of the railroad restores access to Berdyansk. According to British intelligence, this rail link passes through an area that can be hit by Ukrainian long-range precision-guided systems.
It is noted that the new rail line south of Donetsk is almost 60 km long and can be built in eight months. As noted in the UK Ministry of Defence review, this construction is almost certainly one of Russia's largest infrastructure projects in the temporarily occupied territories of Ukraine. This line restores the railway connection interrupted by the fighting in the Donetsk region.
At the same time, British intelligence experts believe that one of Russia's goals will be to maintain activity in Mariupol. The British Ministry of Defence has stated that the Russians are likely to want to restore Azovstal and other facilities that were destroyed in 2022.