That question is not as easy to answer as one would imagine. Do you own or have access to a multimeter?
Here is the dilemma. With a power supply too small you will have the same problem you have now. With a power supply too large you run the risk of melting things or even starting a fire if there is a short circuit like from a derailed locomotive. Additionally, your track length, joiner integrity, frequency and length of feeder wires, bus wire length and gauge, and future expansion plans factor into the answer.
You perhaps have read a lot of forum discussions about wire size for bus and feeders. Too often the advice given is based on how much current is needed to operate trains. Better advice is based on the buss and feeders' ability to pass 100% of available power supply current with minimal voltage drop. This approach assures the power supply over-current protection will operate as designed in the event of a short circuit. So, the question that should be asked is how large of a power supply will my wiring arrangement safely permit, not how much power do my trains need. Hence why I ask if you have a multimeter. With a few measurements on your layout we can give you a good and safe answer before you buy a power supply.