Improvements in performance in January, ticket discounts for fed-up passengers and the appointment of a new managing director have saved WMT for now.
However Mr Street insisted the company, which runs trains through the Black Country, Birmingham and Shropshire, would remain "under strict review" and warned the threat of franchise removal remained.
He said he wanted to see evidence January's improvements were not just a one-off, and has also asked the Government to impose a "serious fine" over delays and cancellations last year.
West Midlands Trains have got better since my ultimatum, but it’s still NOT GOOD ENOUGH. So, today I am:— Andy Street (@andy4wm) February 3, 2020
🚨Keeping them under strict review with the continued threat of franchise removal
💰 Asking the Government to impose a serious fine (1/2) pic.twitter.com/NBzl37FC17
In a letter to Transport Secretary Grant Shapps, the West Midlands Mayor said WMT had made significant improvements since issuing his ultimatum.
He said: "But I still have very serious concerns, and so do passengers. An extensive customer survey I had carried out this month found that, of the more than 4,100 passengers quizzed, 65 per cent experienced regular disruption between October and December last year.
"That figure has dropped to 34 per cent in January - but that's evidently still way too high and remains unacceptable.
"Many passengers are also familiar with WMT's staffing problems, and as a result are sceptical that the short-term performance can be sustained.
"February half-term will be the next real test of the staffing situation, and I will not hesitate to ask you to take the franchise away from WMT if last year's problems resurface.
"Passengers also remain furious at the serious overcrowding that continues to plague a number of services.
"I have been told that this will be tackled and that new carriages are coming, but I am yet to see any substantial evidence of this.
"Therefore, with all the above in mind, I will be keeping WMT under regular review with the threat of losing the franchise still very real.
"The improvements we have seen this year are a start, but they definitely do not make up for months of misery.
"There is a very long way to go if WMT want to keep the franchise long-term, and I will be relentless in my pursuit of these continued improvements."