A travel startup is charging a staggering $18,500 for a six-person charter aboard a private jet–style rail car hooked onto an Amtrak train, so passengers can travel in “ultimate luxury” with a chef, steward and other out-of-this-world amenities. Philadelphia-based Lunatrain is offering the high-end service on two Amtrak routes — a 36-hour California Zephyr run...