Packs cannot be downloaded outright due restrictions on version available on the DLS and repairs to outdated (usually abandoned) assets.
Policy is to have the packs there in order to supply missing dependencies for assets already installed.
You need an earlier version of Trainz to get access to the Chicago Traction Pack. It is not part of TANE.
The policy relates to versions installed and assets part thereof.
I have no idea what the Chicago Traction Pack is but if you download these from the DLS
<kuid2:522774:100023:2> Chicago Metro 2.2
<kuid:522774:100545> Chicago Metro V2 CTA Session 1
<kuid:522774:100398> Chicago Metro V2 CTA Session 2
You get this
Which is most of Chicago and includes sniper297's CTA 4000s and CAT 6000s
Works fine in TANE so far as I can tell. Looks a bit dated but no major flaws I've found. (Traction motor in background is not included but is available on the DLS)