Strategic European Horse Racing features one aspect, that most sims don't deal with, and that is training, so I will give a strategic look at training, that you can either follow or ignore.

First some general aspects.

Each week you have to decide, if you want to train your horse, or enter it in a race, training is only for 2 and 3yr olds, as 4yr olds
are kind of "complete" they will train automaticly, to keep their abilities.

Before deciding what to train, you need to know, what you can train, the answer is simple, everything, every ability can be trained.

Consistency, Recovery and Aging are considered perfect at 100. Grounds and Disctances can be trained further, to improve horses in special categories.

You can name 2 abilities each week to train.
You will improve your abilities by 5 to 10 points in every training session, how many points you gain is random.
If a distance rating is higher 120 or a ground rating is higher then 100, the effect in training will only be 1 to 6 points.

So let's look at the category, and investigate, why and when you should train them.

Let's start with the basic categorys.

Consistency, this is one of the most important things, as it marks the number of % your horse will put into a race, also a 50% horse could perform at 100%, it is less likely, the consistency number gives the minimum performance a horse will have in a race. So if it is 50, the horse will give between 50% and 100% of it's potential,
as this won't satisfy you, try to train consistency early.

Recovery, recovery is another important ability, that every horse starts at with 50, once it gains 15 points, his rest period is reduced by 1 week, the rest period is the time a horse will need to train, before being able to run again, that means, if you horse runs Week 1, and has a recovery of 50, it will not be able to run before week 5, without risking a higher chance of  injury. At a recovery of 90, your horse can run every week. So, if you want a horse, that can run often, train recovery early.

Injury, all horses start already with a rather high injury rate, the injury number gives the percentage, that a horse will complete a race without getting injured,
this means at 75, your horse has a 75% chance of running without injury, and has a chance of 25% to get injured. If it is over 90, you are pretty save, I think there are more important aspects to train, unless it is less than 80.

Aging, this is perhaps the most critical ability, as it has no effect at the ages of 2 and 3, but has to be trained at that age. A horse with less than 50 points will lose ability points every time it runs at the age of 4 and up. while horses with 50-74 points, will start loosing ability when they turn 5, and horses with 75 points plus, will get one more season, and will start to loose points at the age of 6. This ability is I think a make it good or leave it training option, if you want your horse to be succesful as a 2 and 3yr old, you can ignore this ability, but if you aim at winning at the ages of 4 and 5, which is the best age for long and stayer type of horses,
you should train it at least up to 50.

Grounds(Firm,Good,Soft,Heavy): Depending on what type of horse you get, you might look at improving your horse on one or two more grounds, as
there is a lot of rain in europe, especially in the early and late season, you can expect all kinds of ground, with none being favored, also heavy is not likely to happen during the summer, in April or October, it is always a possibility. I suggest training for at least a second ground, if your horse only can perform good on one ground,
as there are sometimes in between situations, where two grounds are needed.

Disctances(Sprint,Mile,Middle,Long,Stay): Some will concentrate on one distance, some will even try to be good at 3 distances, a general rule is, to have at least points in two grounds, as it improves your horse also for in between distances, imagine a sprinter with 100, he can only perform at 50 in 7f races, but if you train his mile ability to only 50, he will not win races at a mile, but at 7f, he already has 75 points, and might beat sprint specialists at that distance.

A General Rule might be, the more grounds you cover, the more you can concentrate on only one distance, as your selection of race, will only be determined by distance not by ground. If you are only good on one ground, try training either for a second ground, or a second distance, to gain more possible races.

So every ability is important, but what matters even more is the combination, because a superstar at one distance, that has no consistency, or needs to long to recover, will make it tougher to win races.