Although negotiations have now begun with the preferred bidders, TUSRC has not ruled out the participation of the other bidders, which are now preparing revised proposals. The Norinco bid of $630 million is for the technically easier but more urgent of the two lines, while the IDRO bid of $900 million is for the challenging north-south line that is planned for longer-term development.

Although Norinco is likely to carry out line 4 with local contractors, IDRO is expected to seek major international companies to join it on line 3, which climbs several hundred metres from south Tehran to the elevated north of the city. It is understood to be in negotiations with other bidders on the project, including Germany's Siemens, to join as the foreign partner. Its $900 million price for the line was well below that submitted by other bidders, making it likely the project will be subsidised heavily by the Industry Ministry, of which IDRO is a branch.

One of the key issues for TUSRC is the manufacture of rolling stock, which the company insists is carried out locally. Norinco has signed a joint venture agreement with TUSRC to establish local manufacturing facilities, but the proposals still require a licence from the Industry Ministry.

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