London Underground's operational development and upgrades director, Nigel Holness, said he was disappointed at the threat of industrial action over new trains.
He said good progress had been made in talks with the RMT on Wednesday and a further meeting was planned today.
"I would urge the RMT to work with us to bring about these improvements and remove the completely unnecessary threat of pointless disruption from hanging over our customers,"
On the Piccadilly line ballot, Phil O'Hare, LU's Piccadilly line manager, said he was hopeful the issues with the RMT could be resolved.
He said there had been two "very positive meetings" with the union, and management was "due to meet with the union's regional organiser next week to finalise an agreement".
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The Metropolitan Police made the arrests after three men suffered stab wounds at Hyde Park near Knightsbridge.
"Based on witness statements and having reviewed the CCTV footage, there is no evidence of anyone spitting in this incident," said police.
Their cat Rothko, resplendent in a bow tie, was the only witness present.