There are three major brands of wooden railway available:
Of the three, BRIO products are of the highest quality; BRIO pieces exceed the most exacting standards. Thomas & Friends products are improving, and are of particular interest to youngsters inspired by the books, movie or television series. Some Eichhorn products, meanwhile, present a cost-effective alternative to their BRIO counterparts.
Thomas products look and feel significantly different to those made by competitors - the Thomas track, for example, is illustrated and looks out of place next to BRIO and Eichhorn track. As such it is probably not a good idea to mix and match Thomas products with other brands.
There is also a wide range of lesser-known brands, such as the Early Learning Centre (ELC) products in the United Kingdom. Generally these products should be evaluated carefully as quality varies considerably, although this consideration is not particularly important when purchasing accessory products (see below).
BRIO track is expensive and in some areas it is possible to save significant amounts of money by using Eichhorn products. Eichhorn track is roughly 2/3 of the price of BRIO track, and the quality is generally excellent.
Unfortunately, most Eichhorn track products beyond basic straight and curved pieces should be avoided in a mixed environment. Eichhorn parallel switching tracks are problematic, as the width between the two tracks at the duplicated end of the piece is significantly wider than BRIO or Thomas equivalents. This problem also exists with duplicated Eichhorn bridges, although here the impact on a layout is much less serious. Eichhorn pylons. meanwhile, are slightly shorter than their BRIO and Thomas equivalents - as a result, some trains are unable to fit underneath raised track.
(I believe that Maxim track is also of reasonable quality, but have not had the opportunity to evaluate it.)
Engines and Carriages
Unless you are collecting Thomas & Friends rolling stock specifically, I recommend purchasing only genuine BRIO products. Products other than those made by these two brands are almost always sufficiently inferior to preclude their use. Engines and carriages made by Eichhorn, for example, use wooden instead of plastic wheels, and are not as solidly constructed as their BRIO counterparts.
Each of the major brands is guilty of severely limiting their range of accessory products, including stations, buildings, trees, cars and more. Because there is no need to directly match track pieces or rolling stock, you should feel free to mix and match accessories from as many sources as you wish - the only limitation is the scale, which should obviously be similar. An excellent selection of compatible accessories may be found on eBay.
If you want an electric train, go and buy an electric train set. Battery-powered engines defy the original purpose of the wooden railway concept, to help stimulate the mind. Given the fact that most children start out with relatively simple layouts, battery-powered engines help to limit the imagination - watching a train go round a circle by itself quickly becomes boring, while actually driving it is an entirely different matter. Battery-operated engines also require special switching tracks (points), and in the case of BRIO, new "Smart Track" for full functionality.