The cost of taking a train from San Diego to Los Angeles can vary depending on several factors, including the type of train service, the class of service, the time of booking, and any discounts or promotions available. Here are some general guidelines:

1. Amtrak Pacific Surfliner: This is a popular train route that connects San Diego and Los Angeles along the scenic coastal route. The ticket prices for the Pacific Surfliner can vary widely. As of my last knowledge update in September 2021, one-way adult fares in standard class (economy) typically range from approximately $37 to $47. However, prices can be higher for business class or first-class seats, which offer additional amenities.

2. Booking in Advance: Train ticket prices can vary based on availability, and they tend to increase as the departure date approaches. Booking your tickets well in advance can often result in lower fares. Amtrak offers various discounts for seniors, children, students, and military personnel, so be sure to check if you qualify for any of these discounts.

3. Peak vs. Off-Peak: Train fares may also vary depending on whether you travel during peak or off-peak times. Peak hours are typically during weekday mornings and evenings when commuter traffic is higher. Traveling during off-peak hours may offer cost savings.

4. Additional Costs: Keep in mind that the base ticket price may not include additional costs such as seat reservations, upgrades to higher classes, or any onboard amenities you choose to purchase.

5. Promotions and Specials: Amtrak occasionally offers promotions, special deals, and discounts, especially for travelers who book early or for specific events. It's a good idea to check the Amtrak website for any ongoing promotions or offers.

Please note that train ticket prices can change over time, and it's advisable to check the latest fares and availability on the Amtrak website or contact Amtrak directly for the most up-to-date information and pricing for the San Diego to Los Angeles route. Additionally, factors such as the class of service, time of travel, and any applicable discounts will influence the final cost of your train journey.