The cost of building a 119-mile section of the California bullet train in the Central Valley is projected to increase by $1.8 billion, taking the total to $12.4 billion, according an internal draft report by the state rail authority’s staff. The estimate was made in preparation for a project update that the rail authority is scheduled to release to the Legislature on Wednesday, detailing a plan by Gov. Gavin Newsom to build a partial high-speed system running from Bakersfield to Merced.