[susan53]

No difference as far as I can see if you are referring to dates which are still future at the time of writing/speaking - eg
The law takes effect as of tomorrow / The law takes effect from tomorrow
The other possibility is as from.
The law takes effect as from tomorrow.

The same is true if the event is completely past :
The law took effect as of January 1st, but was revoked three months later. / The law took effect from January 1st, but was revoked three months later.
The other possibility is as from.
The law took effect as from January 1st, but was revoked three months later.

The problem occurs when the event started in the past but is still true in the present - as in your sentence. Now from would have to be replaced by since, and a perfect verb used, though as of/from plus the present sounds just about possible - though, as in your sentence a bit odd. So :

1) ?? As of last week, I'm working at home
2) ?? As from last week, I'm working at home
3)* From last week I'm working at home
4) Since last week I've been working at home.

If you check with a concordancer, you'll find plenty of examples of the patterns in 1/2 and 4, but not of 3. So, adding in a couple of lexical changes which I think also improve it, your sentence could be...

A) The railway network of France, which as of 2008 totals 29,473 kilometres in length, is...
B) The railway network of France, which since 2008 has totalled 29,473 kilometres in length, is..
C)The railway network of France, which since 2008 totals 29,473 kilometres in length, is..

B and C both sound a bit odd to me though - in this context I want a simple verb, expressing a permanent fact rather than the perfect which just talks about past to present. But the simple verb in C sounds odd with since. But then, so does as of, even though it seems just acceptable.

Personally I'd rewrite the whole thing :
In 2008, the French rail network reached its current extent of 29,473 kilometres in length...