eBay has a hard enough time taking down all the obviously fraudulent listings that are put up every day by scammers; I doubt that keyword spamming is a high priority target for removal.
You can certainly report listings that use keywords in an inappropriate manner if it violates policy, but eBay is unlikely to take action unless the violation is particularly blatant or unless eBay receives many independent complaints.
You may be better off altering the way you search to avoid such listings. If you are searching for toy trains, try limiting the category to Toys & Hobbies, or possibly Antiques or Collectibles. Changing the category may allow you to use category specific filters that apply to the item specifics of the listing. Adding extra keywords to your search can help remove some unwanted results, so you do not have to wade through those extra titles when scanning results.
A little time spent honing your search can save a lot of time going through the results, particularly if you use a search more than once. Setting a price range can eliminate many items that you are not interested in. If you know you only want a new or used item, that can cut your results down significantly.
In some cases, the seller may be entirely blameless; eBay will often use keyword substitution, or may direct the search into a particular category based on the keywords you have used. To bypass these features, you can try putting one of your search terms in quotes to search the literal term without substitution, or use an exclusion term preceded by a minus sign to weed out results with those keywords.
Keep in mind that eBay's search algorithms are based on text matching; the computer does not know what a toy train is. It does have lots of data about what users searching using that term wound up looking at and ultimately buying, though. And that can affect what items turn up in results.
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