Train transformers have one pair of screws for each output, which is generally enough for a simple layout, but once you have more than one accessory or building with lights in it, you’ll find it’s difficult to attach all of the wires to the transformer posts.
You can get more on the cheap by repurposing ground bus bars, intended for circuit breaker panels. These are tinplated aluminum bars with screws in them. Depending on the capacity and the brand, they can cost as little as $3-$4. You can just use whatever brand is cheapest, which, in my experience, is usually Eaton. Buy one bar for each of the transformer posts you use for accessories, then loosen one screw on the bar, run a wire from one of your transformer’s accessory posts to one bar and tighten the screw back down, then repeat the process with your transformer’s other accessory post.
Then when you go to connect lights or accessories, simply loosen an unused screw on each bar, connect a wire, then connect the other end to the posts on the accessory.
There will be two positions on the bar that come empty. Don’t try to use the screws that come with them to attach them to your train table, as the machine screws won’t screw easily into wood. Attach them to the train table with two #6 wood screws that are at least .75 inches long.
Some hobbyists use grounding bars to connect track as well. You’ll use less wire if you use a loop, but some command control systems, such as MTH DCS, don’t like loops, so a pair of ground bars is the most economical way to wire the track feeds for an MTH layout. For Lionel command control systems, traditional Lionel or Marx or American Flyer, traditional DC wiring used for other scales, or even DCC, you can use a loop, so you only need bus bars for accessories and lighting unless you find it easier to use the bars.
Some people express concern about aluminum, since we got into a lot of trouble wiring houses with aluminum wire in the 70s. Aluminum is actually a pretty good electrical conductor; it’s aluminum oxide that isn’t. Plating the aluminum with tin protects against aluminum oxide from forming.