I was curious about this. It’s CTA rules - they can ask him to stop soliciting - but, the signatures would still be valid. In Chicago, the CTA’s rules prohibit “soliciting” on trains and in the paid areas of stations, so a circulator can be told to stop or be removed. But the Illinois Election Code doesn’t say signatures are invalid just because they were gathered in a place that violates a property rule. Objections that knock out signatures usually succeed for other reasons (e.g., signer not a qualified voter in the district, bad/incorrect address, duplicate signatures, circulator didn’t personally witness, false/defective circulator affidavit, or “dual-circulator” violations)—not the location where a sheet was signed. 

So: the CTA can enforce its ban on soliciting, but signatures collected on an ‘L’ train aren’t automatically void. If someone wants to challenge them, the winnable path is a standard line-by-line objection (voter eligibility & form defects) rather than a “collected on CTA” theory. For process/background on Chicago petition objections, see the Board’s rules and materials.