Actually, it was Big Four - Leland Stanford, Charles Crocker, Collis Huntington and Mark Hopkins. They all built huge mansions in San Francisco, for the most part destroyed in the 1906 Earthquake and Fire but parts rebuilt as hotels.
Your link about James Hill is a good one. So okay, some railroad tycoons are not robber barons! The Big Four were ruthless crooks. But they were largely responsible for the rapid growth in California after the Civil War.
Government officials were the real crooks, without them the barons wouldn’t be able to rip off the taxpayers.