The trolley carts might not be effective for actually making converts. But, the trolley witnessing is effective in conforming to laws in countries where "preaching" is restricted.
In Russia, it was the trolley witnessing that got JWs acquitted in court.
http://www.forum18.org/archive.php?article_id=2100
By contrast, at least two of the eight Jehovah's Witnesses stopped by police as they offered religious literature to passers-by on the streets of Simferopol on 2 July were acquitted in court, according to court records.
Police stopped the Jehovah's Witnesses as they were offering religious literature from mobile stands, according to court decisions seen by Forum 18. Officers claimed they were violating Russia's June 2004 Demonstrations Law. They drew up records of an offence under Administrative Code Article 20.2, Part 1. The cases were then passed to court.
At separate hearings on 30 July in front of Judge Aleksei Tikhopoi at Simferopol's Kiev District Court, both O. Shchelokova and E. Lyskova insisted that their right to offer religious literature is protected by the Russian Constitution and the European Convention for the Protection of Human Rights and Fundamental Freedoms.
Shchelokova's lawyer told the Judge in her case that "during the religious activity, no slogans or calls of any kind were made, no agitation, propaganda or imposition of any religious views or ideas was conducted, banned religious literature was not distributed, no financial collections to support a religious organisation were undertaken, and no loudspeakers were used. The activity of her client did not form a danger to those around, and did not create a real threat of harm to the object being protected."
In choosing to acquit Shchelokova and Lyskova of any wrongdoing, Judge Tikhopoi specifically acknowledged individuals' right to freedom of religion or belief laid out in Article 9 of the European Convention.
Effective? Yes. In certain countries, trolleys are the only effective way that the WTS can establish a presence.