WASHINGTON—U.S. passenger railroad Amtrak said on Wednesday that Chief Executive Bill Flynn will retire next month and be replaced by its president as it begins a big expansion of operations funded by Congress. Flynn, 68, has overseen the railroad since April 2020 and steered it through the COVID-19 crisis and a massive falloff in demand. Amtrak President Stephen Gardner will succeed him effective Jan. 17, becoming the company’s fifth CEO since 2016. Congress has approved $66 billion for rail as part of a massive infrastructure bill, with Amtrak receiving $22 billion and setting aside $36 billion for competitive grants. Amtrak is adding service, upgrading key stations and has a contract with Siemens Mobility Inc to manufacture a new fleet of up to 83 multi-powered trains and options for up to 130 additional trains. Flynn will continue as a senior adviser to Gardner and the company through Sept. 30, 2022. Amtrak …