Booking trains with specific seat preferences (window, direction, etc.)?

The official booking system doesn't usually let you choose exact seat numbers in advance for most trains. You get a reserved seat in a specific class and carriage. For the best experience on scenic routes, book 2nd class reserved and try to board early to choose your spot. Some people prefer the left or right side depending on direction (views of the valley vs mountains). The observation car on some trains has fixed big windows. For the classic Kandy-Ella journey, many travelers say the regular 2nd class carriages are fine and less crowded with photographers than the observation car.