The flood of buy orders for Shanghai shares through the Hong Kong exchange link slowed to a trickle two days after the program's debut.

Net purchases of mainland equities by global investors totaled 2.6 billion yuan ($425 million) on Wednesday, down from about 4.8 billion yuan on Tuesday and the maximum 13 billion yuan on Monday. Hong Kong stock buying slowed 68 percent from Tuesday to 253 million yuan. The Shanghai Composite Index slipped 0.2 percent and shares of Hong Kong Exchanges & Clearing Ltd posted the biggest three-day drop since 2011.

The tumble in demand on Tuesday spurred investment bank CLSA Ltd to call the program a "Ghost Train", a reference to the so-called through train plan to let mainland investors buy Hong Kong shares, which was originally proposed in 2007. That proposal sent the Hang Seng Index surging before being abandoned.

While CLSA analysts said it was too early to judge the sustainable level of trading through the link, they said usage so far has been disappointing and profit projections for Hong Kong's bourse may be cut.

"While the bulls will point to rumors that many investors are waiting on the sidelines, the bears will point to the already huge visible fall in demand," said Marcus Liu, a Hong Kong-based analyst at CLSA.

"Our expectations on Stock Connect are at the bottom end of consensus."

Hao Hong, a managing director for research at Bocom International Holdings Co, called the debut an "anticlimax", while Yuliang Chang, the chief China and Hong Kong equities strategist at Deutsche Bank AG, said initial flows have been disappointing on both sides.

One reason for the weak demand is that share prices had already surged in anticipation of the link, said Wu Kan, a money manager at Shanghai-based Dragon Life Insurance Co, which oversees about $3.3 billion.

The Shanghai index reached a three-year high on Nov 12, while the Hang Seng index of Hong Kong shares traded at the highest in almost two months on Friday.

"News of Stock Connect was already fully digested and priced in," Wu said. "Investors won't enter the markets to buy shares at such levels now."

While the link has "huge" potential, a concern that may be curbing participation is a requirement for international investors to deliver stocks to a broker before markets open if they plan to sell in Shanghai, said Nick Ronalds, managing director for equities at the Asia Securities Industry & Financial Markets Association. Most major markets let investors transfer shares after placing the sell order.

Wednesday's net purchases of Shanghai stocks, known as Northbound flows, amounted to about 20 percent of the 13 billion yuan daily quota. For Southbound purchases of Hong Kong shares, about 2.4 percent of the 10.5 billion yuan quota was taken up.