Book your tickets online at the Rail Europe Web site (see the Resources section below). You can also call Rail Europe at (888) 382-RAIL (382-7245). Expect midrange prices.
Consider your options before your book your seats. The trip will take at least 12 hours. Your most direct route is to take a train from Coimbra to Lisbon's Oriente station, where you can hop onto a train to a Spanish border city like Irun. From Irun, take the AVE train system into Madrid.
Choose your service class, including sleeper selections. Sleeper cars are available on the longer legs of your journey, and though they cost substantially more, there's not a more relaxing way to travel by train. Purchase a regular seat, and then upgrade to a sleeper or couchette (cars with beds shared by up to 6 individuals) for any leg of the journey.
Book nighttime tickets to save additional money, since daytime fares are usually pricier.
Reserve each leg of your journey individually, unless you purchase a Eurail pass.
Take a taxi or regional train to the main station in the center of Coimbra. From there, travel to Lisbon.
Change trains at the Lisbon Oriente station and go to the platform with trains bound for Spain.
Check your ticket to verify seat location, and board in the designated area.
Hand tickets to the conductor when requested. Remember, the conductor may visit several times, so keep tickets handy throughout your journey.