DT12 is the only studio album with musical contributions by all members..
Aren't most DT songs credited to the whole band for music?
Short answer: no..
Very long answer: all DT albums from WDADU to SC -save SDoIT and ToT- credit every song to "Dream Theater", except for some songs that are credited to one band member: WFS and SDV (KM), TSM, Regression and TMW (JP).. In SDoIT, ToT and BC&SL, they credited all songs to "JM, JP, MP and JR", except Vacant (JM and JR) and Wither (JP).. So, no many know this, but the guys (or maybe only Portnoy) made that change in those albums and credited
"who actually composed the songs" because they (or he) wanted more participation from James in the making process of the songs, since he seemed to be losing interest even in being there while they were
"creating the music to which he'll actually sing over" (these are not exact phrases, but very aproximated to what the interviews in the Lifting Shadows biography say)..
Moreover, another little issue about this was that James in the interviews was actually giving the impression that he was composing the music too, so for all of this they had a reunion in which they ask him to get involve a bit more in the making process and he thought it was fair enough.. So he did and throughout ToT, 8V and SC he was always there at least listening and watching them, and slightly helping to create the songs.. Still, the credits excluded him (and justly).. But since ADToE he kind of replaced Portnoy and started adding actual sections to some songs (some more evident than others, like the post-chorus of BMU,BMD), and the album kept the BC&SL way of crediting, but now would actually make more sense, so to speak, since there are all kind of different combinations of authors..
And we finally got to DT12, the last album until now in having discriminated credits, since TA is entirely credited to JP and JR.. In the self-titled, there are six songs that are, for the first time, credited to every single band member, and not just to "Dream Theater", like those three albums in which James didn't participate and so the "DT" corresponded only to the four guys.. (note that seven songs off WDADU are also credited to "DT", but I think in this one Dominici did contribute to the music, because TYJ excludes him, unlike the rest of the instrumentals that don't exclude James -they don't not because he has co-created the track, but because he didn't co-create any of the songs from I&W to BC&SL)..
So, out of all this mess, I have another one:
there are only three songs in the entire DT catalog that are not musically credited to John Petrucci (Wait for Sleep, Space Dye Vest and Vacant)..