I think the basses have been pretty well covered here, but will triple down on making sure you have enough power feeders to avoid drop-outs around your track. Also make sure each piece of track is making a good connection to the next. I didn't see a mention of what type of track you are using, and the process of insuring a solid connection between sections varies a little between types. For the traditional tubular "O" or "O27" track you sometimes get a little rust on the pins or inside the "tube", the inside of the rail where the pins plug into. This is fixes easily enough with a tiny piece of fine grit sand paper rolled up to fit, a small round file, or if you can find a metal-bristle pipe cleaner, they work pretty well. The ends of the track can also be loose, and there are folks that sell special pliers to tighten the ends of the track... I've always used a pair of needle nose pliers along the flat sides, from the end of the tack.
Other things to look for are simply dirty track and dirty wheels. using the search feature of the forum you will find countless threads on cleaning track and wheels.
even with the undersized transformer, your locomotive should run consistently in all area's of the track...well likely just a bit slower around tight curves, as they put more friction on the wheels.
Once the track problems are solved, I second the CW80 as an inexpensive choice to run a couple accessories and 1 or 2 engines. MTH's Z1000 is a comparable choice to the CW-80, and preference goes back and forth on which one people think is better. I'll take the CW80 because it's circuit protection is better, and I don't have to worry about unfused shorts that can occur with the Z1000 between track and accessory outputs. The GW180 looks like a really nice transformer as well, but I just don't know a lot about them, and they are very rarely talked about here. I think they just don't make sense cost-wise for a lot of folks.
For your post-war style transformers, I find the ZW to be my favorite, followed by the KW. These are really great work-horse transformers, but they do require the addition of a quality circuit breaker to protect the wiring in newer locomotives and such if you want to be safe with them. You could also go with something like the 1033 which is somewhat comparable to the CW80 for output power if you need something small.
On the top end products, I don't think the ZW-L can't (edited typo) be beat, but it's price is as heavy as the transformer is. While there are some problems with it, the older ZW-C is a fair transformer and is expandable to use with 1 to 4 of the 180 watt powerhouse bricks, and can also be used to vary track power from TMCC if you ever expand in that direction. I'm not a big fan of the MTH Z4000, mostly because it looks like someone got drunk, smashed their hands with a sledge hammer, then drew a picture of a ZW. Ascetic preferences aside, it is a fairly good transformer, even if it is getting dated. The newer lionel products offer much better short-circuit protection.
Other considerations:
As others mentioned, buy a larger transformer than you think you need. You don't want to have to buy another one the next time you expand your layout or buy an engine or accessory that draws some more power.
Circuit breakers: The circuit breakers inside most transformers are designed to protect the transformer from damage in a short, not to protect the trains or track wiring! Unless you are using a current generation lionel transformer with fold-back or fast-acting electronic breakers (CW-80, GW-180, PH-180, ZW-L) you are going to want to add circuit breakers between the transformer output and any track or accessories connected. You want to use the lowest value circuit breaker you can get away with here. For one engine, something in the 4-5 amp range should be more than enough, and I wouldn't recommend anything over a 7 amp breaker even with large transformers for someone that does not know exactly what they are doing.
TVS diodes: TVS's are very inexpensive parts, <$.50 each that snub out spikes of high voltage. When things like switch machines and uncoupling tracks are turned off they can send a high voltage spike back through the track and transformer (among other things). This is not a problem for old post war locomotives but for newer ones these spikes can damage the electronic components in them. The simplest use of the TVS diode is to place it right on the transformer, one connected from each output post to a U post. This will give you much more protection than nothing, but you can't have too many TVS's on your layout. Many folks recommend placing one across the feeder wires at every feeder connection to the track. Some folks even install them right inside the locomotives for optimal protection.
Pure Sine Wave vs. Chopped Wave transformers: You may catch this one in conversation about transformers from time to time. It is only an issue with running some of the very early electronic controlled locomotives. I believe some with QSI electronics and some of MTH's original ProtoSound locomotives. Any modern Locomotive should run fine with chopped wave transformers, and I believe Lionel even claims that that their engines prefer chopped wave transformers. TMCC, Legacy, and DCS will all run just fine from Chopped waves within their recommended operating voltages.
JGL
And if something can happen, sooner or later, somewhere it will.
