Although I haven't tested this yet, I'm not sure that Bachmann's On30 trains will withstand the intense attention of a couple large dogs! You might want to make sure they stay out of the reach of your pets. Lionel O gauge trains probably would survive a close encounter, although they're not usually to scale, and are quite a bit larger than the On30 trains.
As far as Large Scale trains being pricey, you get what you pay for. LGB trains are among the best in the industry, bar none. They're designed to handle just about any weather, except maybe golf-ball sized hailstones! Aristo-Craft trains are almost as good, and Bachmann are somewhat lower down the quality chain, though their prices are more reasonable for starting out.
Pat