I think this would be the neatest, clearest and most accurate way of putting it. It's also quite close to the original, the main change being to reduce the rather clumsy formulation of "los estados a los que pertenecen sus sindicatos", with the repetition of "sindicatos", to "their respective countries", which I think covers it.

The key thing here is what "porcentaje de representatividad" actually means: percentage of what? It means the percentage of eligible workers: how many of those who could belong to these unions actually belong to them.

The ALE is a German-based federation of 16 independent national train-drivers' unions. One of them is Spanish, actually: SEMAF (Sindicato Español de Maquinistas y Ayudantes Ferroviarios). So the workers elegible to join these unions are specifically train drivers or locomotive drivers and assistant drivers, and (I think) some other on-board train workers. SEMAF's porcentaje de representatividad is the percentage of all such workers (only such workers) in Spain who belong to it.
http://www.semaf.net/portal/n1_lks.php?idamb=31
http://www.ale.li/index.php?id=2&L=1&N=1

So it's not a percentage of train operators. Nor, in fact, is it a percentage of train union representation, because that would mean the proportion of all unionised workers (implicitly those in the relevant category) who belong to SEMAF, but it actually means the percentage of all train drivers, unionised or non-unionised. And then "train drivers" alone is not totally accurate, because apparently some other workers apart from train drivers can join SEMAF and other ALE unions if they choose. So I would really rather not have to express what it is a percentage of.

You could say "percentage of elegible workers"; that would be safe enough. In other words, "The percentage of elegible (railway) workers represented by ALE-affiliated unions in their respective countries is ...%" would be OK. But I'd prefer to use "representation rate", which means exactly what we want.

"Three unions are active at the Cyprus Telecommunications Authority, the Free Pancyprian Organisation of Telecommunications Employees (EPOET), affiliated to the Cyprus Workers' Confederation (Συνομοσπονδία Εργαζομένων Κύπρου ΣΕΚ), which represents around 60% of employees, the Local Authority Workers’ and Employees’ Trade Union (SIDIKEK), affiliated to the Pancyprian Federation of Labour (Παγκύπρια Εργατική Ομοσπονδία ΠΕΟ), and the Pancyprian Independent Employees’ Union (ΠΑΣΕ) of ΑΤΗΚ, with a representation rate of around 14%."
http://www.eurofound.europa.eu/eiro/studies/tn0606017s/cy060...

This means that 14% of Cypriot telecommunications workers belong to the Pancyprian Independent Employees’ Union.

" For third year, union representation rate falls
About 11% of Gannett's U.S. employees were represented by labor unions by the end of last year, according to the recently released annual 10-K report to federal securities regulators. That's down from 12% in 2010 and 14% in 2009."
http://gannettblog.blogspot.com.es/2012/03/for-third-consecu...

"Vodacom questioned the representation rate of the CWU, saying that in “Vodacom SA the CWU currently has representation of less than 10% and not the agreed threshold of 30% which would qualify them to enter into a collective agreement.”"
http://mybroadband.co.za/nephp/5855.html

And by the way, I think David and Pat are right that "ALE unions" alone would do, but I would prefer "ALE-affiliated".

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Note added at 15 hrs (2012-12-20 12:42:53 GMT)
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Sorry about "elegible": interference from Spanish. Read "eligible".